refactor passing down of watch flags to avoid bugs

[miparnisari]

Originally posted by @miparnisari in #2859 (review)

We've seen two instances (one: #2636, the other caught in a PR #2859) where a server flag wasn't passed down properly to the datastore's implementation of the server method being called.

this is because the process of passing down a flag involves too many changes and it is VERY to miss a step.

i think a refactor is due.

proposal for Watch:

• split datastore.WatchOptions struct to ServerWatchOptions and ClientWatchOptions
• add a new datastore.WatchOptionsDefaults interface method that must be overriden per datastore
• introduce a common fn merge(serverOptions, defaultDsOptions, clientWatchOptions) -> ClientAndServerOptions. also, serverWatchOptions are the same no matter the datastore, so they don't need to live in the cds or memdb implementations, they could live inside watchServer,
• have server.Watch() call the merge function
• then we change the datastore.Watch interface to take ClientAndServerOptions

This proposal could later be extended to other APIs.