kv: append log entries outside of raft state machine loop

[nvanbenschoten]

Extracted from #17500.

After #94165, raft log entry disk writes are asynchronous with respect to the raft state machine loop. However, the (non-durable) engine access for raft log appends is still performed inline. The async storage writes interface permits us to extract all of this work onto a separate goroutine.

This would provide two benefits:

1. faster state machine loop iteration => less interference between entries => lower latency
2. larger append batches => more efficient log appends => higher throughput

Jira issue: CRDB-23188

Epic CRDB-39898

[blathers-crl[bot]]

cc @cockroachdb/replication