raft: why the raft log does not use a separate rocksdb instance?

[jiadexin]

Is your feature request related to a problem? Please describe.
why the raft log does not use a separate rocksdb instance?

Describe the solution you'd like
The current raft log instance and the kv data instance both share the same rocksdb. I wonder why the raft log does not use a separate rocksdb instance? I think this performance should be better.

Describe alternatives you've considered
The raft log use a separate rocksdb instance

[irfansharif]

We are actually looking to separate out our raft storage from the shared rocksdb/pebble instance, we're just not quite all the way there yet. The current next step is #39182, and something I'm currently working, before we can fully flesh out the work that'd go into separating out raft storage in its own storage device.