Support ReplicaFetcherThread fetch log async

Currently, in `ReplicaFetchThread#processFetchLogRequest()`, we will block and wait for the fetch log request to complete. This can be changed to an asynchronous approach to improve the utilization of the fetch threads.

Locate ReplicaFetchThread#processFetchLogRequest() and read the surrounding fetch-thread request lifecycle first. Determine how the fetch log request currently waits for completion and identify the behavior that must remain correct when it is handled asynchronously. Done means the request no longer blocks the fetch thread while preserving correct fetch processing.

Written by the indexing model from the issue text.