chore: Cache proven chain tip in State

[sergerad]

Closes #1841.

Cache proven-in-sequence tip to avoid repeated DB queries

Adds a watch::channel-based cache for the proven-in-sequence block number, so RPC endpoints requesting Finality::Proven no longer hit the database on every call.

Changes

• Introduces a Finality enum (Committed / Proven) in the store's State
• Replaces State::latest_block_num() with State::chain_tip(finality) which returns the committed tip (from the in-memory MMR) or the proven tip (from the cached watch channel) depending on the requested finality
• State::load() initializes the proven tip from the DB and returns a watch::Sender that the proof scheduler uses to push updates after mark_proven_and_advance_sequence advances the sequence
• Simplifies sync_chain_mmr from a match + DB query into a single chain_tip(finality) call
• Splits up Store::serve() into helper functions spawn_proof_scheduler() and spawn_grpc_servers()

[Mirko-von-Leipzig]

I'm not sure how I feel about defaulting here for Unspecified, but its probably not a train smash.

[SantiagoPittella]

Could we benefit of using AtomicU32 inside State instead of the channel? Considering that we are only borrowing to get the latest value

[Mirko-von-Leipzig]

Could we benefit of using AtomicU32 inside State instead of the channel? Considering that we are only borrowing to get the latest value

Possibly yeah. Though I would insist we new type wrap it so it has a BlockNumber API, and that we get a single writer (non-clone), and a reader (clone/copy).

[sergerad]

Could we benefit of using AtomicU32 inside State instead of the channel? Considering that we are only borrowing to get the latest value

Possibly yeah. Though I would insist we new type wrap it so it has a BlockNumber API, and that we get a single writer (non-clone), and a reader (clone/copy).

Happy to do this in a followup if we think its worth while. Seems a bit more boilerplate for potentially low/no benefit.

EDIT: Updating this in #1857 as it does help with the logic (potentially even necessary)