feat: raft log compaction

This patch is extracted from !478 (merged), but refactored significantly.

1. Implement the raft log compaction
2. Refine and document the behavior of
   • RaftSpaceAccess::entries()
   • raft::Storage::entries()
   • raft::Storage::term()
   • raft::Storage::first_index()
   • raft::Storage::last_index()
3. Test it intensively.
4. Assert that persist_* functions are invoked in a transaction