consider remove the raft gc thread

[glorv]

After introducing raft-engine, the cost of raft log gc task is almost the same as a normal write. Thus, it's unnecessary to still use a separate thread to handle log gc. While the current aync gc approach is unlikely to be a bottleneck, it makes the codebase a litter simpler.

[glorv]

/cc @Connor1996 @overvenus @LykxSassinator @tonyxuqqi what do you think?

[LykxSassinator]

Emmm....Unless we announce that rocksdb-raft will not be maintained in the future.

Currently, if the user still wanna use rocksdb-raft option, this change might have side effects to the performance when gc raft logs.

[Connor1996]

We can first remove the support of rocksdb-raft

[LykxSassinator]

We can first remove the support of rocksdb-raft

+1