[Relayminer] Add ReplayObservable#SubscribeFromBufferEndOffset() method

[bryanchriswhite]

Objective

Explicitly support choosing how much of the replay buffer should be emitted when subscribing to the replay observable.

Origin Document

While working on #517:

See: #546 (comment).

	// SubscribeFromBufferEndOffset returns an observer which is initially notified of
	// values in the replay buffer, starting from the replay buffer index which is
	// equal to replayBufferSize - offset. After this range of the replay buffer is
	// notified, the observer continues to be notified, in real-time, when the publishCh
	// channel receives a value. It also returns the current replayBufferSize.
	// If offset is greater than replayBufferSize or the number of elements it
	// currently contains, the observer is notified of all elements in the replayBuffer,
	// starting from the beginning. Passing 0 for offset is equivalent to calling
	// Subscribe() on a non-replay observable.
	SubscribeFromBufferEndOffset(ctx context.Context, offset int) (_ Observer[V], replayBufferSize int)

Goals

• Support more explicit behavior when consuming replay observables.

Deliverables

• A PR which implements ReplayObservable#SubscribeFromBufferEndOffset()
  • [Observables] chore: add ReplayObservable#SubscribeFromLatestBufferedOffset() #647
• A PR which refactors RelayerSessionsManager#waitForBlock() to use it and ensure that the returned replayBufferSize is sufficient to include the pathBlock, erroring otherwise, e.g.:
  • [Relayminer] refactor: relayerSessionsManager#waitForBlock() #648

    blocksSinceClaimWindowStart := currentHeight - claimWindowStartHeight
    observer, bufferLen := #SubscribeFromBufferEndOffset(ctx, blocksSinceClaimWindowStart)
    if bufferlen < blocksSinceClaimWindowStart {
      // query for blocks between `blocksSinceClaimWindowStart` & `replayBuffer[0]`...
    }

Non-goals / Non-deliverables

• ...

General deliverables

• Comments: Add/update TODOs and comments alongside the source code so it is easier to follow.
• Testing: Add new tests (unit and/or E2E) to the test suite.
• Makefile: Add new targets to the Makefile to make the new functionality easier to use.
• Documentation: Update architectural or development READMEs; use mermaid diagrams where appropriate.

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Creator: @bryanchriswhite
Co-Owners: @red-0ne