storage: Refactor raft.Storage methods

[bdarnell]

storage.Replica currently implements the etcd/raft.Storage interface directly, which exposes methods with externally-imposed names. Some of these methods require locks to be held (e.g. FirstIndex, but they cannot follow our usual convention of a Locked suffix (they also have some very generic-sounding names like Entries). We should define these methods on a new replicaRaftStorage type alias instead of *Replica itself so they are not used with misleading names elsewhere in the storage package. Any methods that need to be accessible within package storage can be defined on *Replica with a name like raftFirstIndexLocked.

See #15935 (comment)

[a6802739]

I could have a see on this part.

[bdarnell]

Go ahead @a6802739