kv: apply log entries outside of raft state machine loop

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 state machine application is still performed inline. The [async storage writes interface](https://github.com/etcd-io/raft/pull/8) permits us to extract all of this work onto a separate goroutine.

This would provide three benefits:
1. faster state machine loop iteration => less interference between entries => lower latency
2. larger apply batches => more efficient state machine application => higher throughput
3. flexible scheduling permits deferred application on followers => bigger batches, see benefit 2

Jira issue: CRDB-23189