compute,storage: reduce the size of cluster commands

[teskje]

ComputeCommand and StorageCommand are enums, so their size is dependent on the size of the largest variant. Both contain some huge variants, in particular UpdateConfiguration and the ones describing dataflows to install, bringing the size of the enums to 3-4 KB. In contrast, the variant we send most frequently, AllowCompaction, is only 40 bytes in size, so when handling an AllowCompaction command, we waste 3-4 KB of space. Commands are stored in histories and channels, so this amount of waste can have an impact on our memory usage.

This PR removes the waste and brings the size of the command enum down to 40 bytes by boxing large fields. This makes code creating commands a bit more noisy, but no way to avoid that I think.

(First two commits are from #32258 and can be ignored here.)

Motivation

• This PR refactors existing code.

Tips for reviewer

I didn't try the reduce the size of the responses here for two reasons:

• They are only 120 bytes in size, so the waste is much smaller than for commands.
• At least for compute, the 120-bytes response is also the one we send most of the time (Frontiers), so moving its data to the heap wouldn't safe much memory overall.

That's not to say we shouldn't try to reduce the memory usage of responses too, just that it's less urgent.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[teskje]

For reference, here are the memory layouts of the two enums before this change:

struct mz_compute_client::protocol::command::ComputeCommand<mz_repr::timestamp::Timestamp>
        size: 3816
        members:
                0[8]    <anon>: u64
                0[3816] <variant part>
                        CreateTimely: <2>
                                0[8]    <padding>
                                8[64]   config: struct mz_cluster_client::client::TimelyConfig
                                72[16]  epoch: struct mz_cluster_client::client::ClusterStartupEpoch
                                88[3728]        <padding>
                        CreateInstance: <3>
                                0[8]    <padding>
                                8[64]   __0: struct mz_compute_client::protocol::command::InstanceConfig
                                72[3744]        <padding>
                        InitializationComplete: <4>
                        AllowWrites: <5>
                        UpdateConfiguration: <0>
                                0[3816] __0: struct mz_compute_client::protocol::command::ComputeParameters
                        CreateDataflow: <7>
                                0[8]    <padding>
                                8[352]  __0: struct mz_compute_types::dataflows::DataflowDescription<mz_compute_types::plan::render_plan::RenderPlan<mz_repr::timestamp::Timestamp>, mz_stora
ge_types::controller::CollectionMetadata, mz_repr::timestamp::Timestamp>
                                360[3456]       <padding>
                        Schedule: <8>
                                0[8]    <padding>
                                8[16]   __0: struct mz_repr::global_id::GlobalId
                                24[3792]        <padding>
                        AllowCompaction: <9>
                                0[8]    <padding>
                                8[16]   id: struct mz_repr::global_id::GlobalId
                                24[24]  frontier: struct timely::progress::frontier::Antichain<mz_repr::timestamp::Timestamp>
                                48[3768]        <padding>
                        Peek: <10>
                                0[8]    <padding>
                                8[544]  __0: struct mz_compute_client::protocol::command::Peek<mz_repr::timestamp::Timestamp>
                                552[3264]       <padding>
                        CancelPeek: <11>
                                0[8]    <padding>
                                8[16]   uuid: struct uuid::Uuid
                                24[3792]        <padding>

struct mz_storage_client::client::StorageCommand<mz_repr::timestamp::Timestamp>
        size: 4352
        members:
                0[8]    <anon>: u64
                0[4352] <variant part>
                        CreateTimely: <2>
                                0[8]    <padding>
                                8[64]   config: struct mz_cluster_client::client::TimelyConfig
                                72[16]  epoch: struct mz_cluster_client::client::ClusterStartupEpoch
                                88[4264]        <padding>
                        InitializationComplete: <3>
                        AllowWrites: <4>
                        UpdateConfiguration: <0>
                                0[4352] __0: struct mz_storage_types::parameters::StorageParameters
                        RunIngestion: <6>
                                0[8]    <padding>
                                8[1800] __0: struct mz_storage_client::client::RunIngestionCommand
                                1808[2544]      <padding>
                        AllowCompaction: <7>
                                0[8]    <padding>
                                8[16]   __0: struct mz_repr::global_id::GlobalId
                                24[24]  __1: struct timely::progress::frontier::Antichain<mz_repr::timestamp::Timestamp>
                                48[4304]        <padding>
                        RunSink: <8>
                                0[8]    <padding>
                                8[2240] __0: struct mz_storage_client::client::RunSinkCommand<mz_repr::timestamp::Timestamp>
                                2248[2104]      <padding>
                        RunOneshotIngestion: <9>
                                0[8]    <padding>
                                8[704]  __0: struct mz_storage_client::client::RunOneshotIngestion
                                712[3640]       <padding>
                        CancelOneshotIngestion: <10>
                                0[8]    <padding>
                                8[16]   __0: struct uuid::Uuid
                                24[4328]        <padding>

[antiguru]

Great!

[teskje]

TFTR!