raft: Boot partition Raft group in replicas proactively

In the newest finding in this comment, it turns out a group leader will actively send AppendLogEntries request to replicas once when the group is first established. This opens up the ability to skip over formal replica bootstrapping by hooking to the networking layer. When the other side receives such messages, it initiates the Raft member joining process. There's a catch in this approach. If the leader halts sending further messages if the replica doesn't respond or join the cluster. However, the leader does receive an acknowledgment from the replica that the initial message is received and stored. It's up to the replica to ignore or handle it.

This issue tracks the work of hooking into the networking layer and initiating corresponding Raft groups.

Prerequisites:

• raft: Wire persistent routing table to Raft manager
• raft: Add membership management to Raft manager

Next:

• raft: Add replication support (without autocompaction) to Raft manager
• raft: Support adding read-only replicas to a partition