This is the beginning of raft protocol implementation. It only supports
log replication and voter state machine. The main difference between
this one and the RFC (besides having voter state machine) is that the
approach taken here is to implement raft as a deterministic state
machine and move all the IO processing away from the main logic.
To do that some changes to RPC interface was required: all verbs are now
one way meaning that sending a request does not wait for a reply and
the reply arrives as a separate message (or not at all, it is safe to
drop packets).
* scylla-dev/raft-v4:
raft: add a short readme file
raft: compile raft tests
raft: add raft tests
raft: Implement log replication and leader election
raft: Introduce raft interface header
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