raft: Wire persistent routing table to Raft manager

!7632 (merged) implements the persistent routing table for the Raft group. The routing table is independent and hasn't been wired to anywhere yet. This issue tracks the work of wiring it to the Raft manager, including:

• Modify the routing table corresponding to node membership events.
• Of course, add tests to cover.

When a Raft member requests to join a group, the leader issues a conf change entry. This entry must be acknowledged by the Raft group. At the committed stage, each group member applies the conf change and adds info of the new member to the routing table. A similar flow applies for node removal and node update.

As of now we don't want to work on membership management, yet, the test suite can issue the conf change entries directly via etcd-raft's Node object. We will improve it in #6643 (closed).

Next:

• raft: Add membership management to Raft manager
• raft: Add replication support (without autocompaction) to Raft manager
• raft: Boot partition Raft group in replicas proactively
• raft: Support adding read-only replicas to a partition