feat: rocksdb migrations

[snowmead]

Summary

Adds a versioned migration framework for managing RocksDB column family schema changes. This enables safe removal of deprecated column families when upgrading existing databases, ensuring backward compatibility with older node installations.

Design

The migration system handles a key RocksDB constraint: you cannot open a database without specifying all existing column families. This means deprecated CFs must be discovered and included when opening, then dropped via migration.

Migration Flow:

1. DB::list_cf() discovers all existing column families (including deprecated ones)
2. Database opens with union of existing + current CFs
3. MigrationRunner checks schema version and runs pending migrations
4. Deprecated CFs are dropped and schema version is updated

Schema version is tracked in a dedicated __schema_version__ column family. Databases without this CF (pre-migration) are treated as version 0.

Notable changes

Migration Framework (client/common/src/migrations/)

• Migration trait defines versioned migrations with deprecated CF lists
• MigrationDescriptor captures migration metadata for runtime use
• MigrationRunner handles version tracking and migration execution
• MigrationError enum for structured error handling
• Comprehensive test suite covering fresh databases, existing databases with deprecated CFs, idempotency, and data preservation

V1 Migration

Drops deprecated MSP respond storage request column families:

• pending_msp_respond_storage_request
• pending_msp_respond_storage_request_left_index
• pending_msp_respond_storage_request_right_index

These were replaced with in-memory queueing in MspHandler.

Unified Database Opening

• TypedRocksDB::open() now handles migrations automatically
• Removed manual DB::open_cf_descriptors() calls from all stores
• Consistent CURRENT_COLUMN_FAMILIES pattern across stores

Stores updated:

• BlockchainServiceStateStore
• DownloadStateStore
• BspPeerManagerStore

Adding New Migrations

// 1. Create client/common/src/migrations/v2.rs
pub struct V2Migration;

impl Migration for V2Migration {
    const VERSION: u32 = 2;

    fn deprecated_column_families() -> &'static [&'static str] {
        &["old_cf_to_remove"]
    }

    fn description() -> &'static str {
        "Remove old_cf_to_remove column family"
    }
}

// 2. Register in MigrationRunner::all_migrations()
vec![
    MigrationDescriptor::new::<v1::V1Migration>(),
    MigrationDescriptor::new::<v2::V2Migration>(),
]