raft: introduce vtab for disk and network

Raft is being moved to a separate library in src/lib. It means,
it can't depend on anything from box/.

The patch makes raft stop using replicaset and journal objects.
They were used to broadcast messages to all the other nodes, and
to persist updates.

Now raft does the same through vtab, which is configured by box.
Broadcast still sends messages via relays, and disk write still
uses the journal. But raft does not depend on any specific journal
or network API.

Part of #5303