This patch introduces partial RAFT implementation. It has only log
replication and leader election support. Snapshotting and configuration
change along with other, smaller features are not yet implemented.
The approach taken by this implementation is to have a deterministic
state machine coded in raft::fsm. What makes the FSM deterministic is
that it does not do any IO by itself. It only takes an input (which may
be a networking message, time tick or new append message), changes its
state and produce an output. The output contains the state that has
to be persisted, messages that need to be sent and entries that may
be applied (in that order). The input and output of the FSM is handled
by raft::server class. It uses raft::rpc interface to send and receive
messages and raft::storage interface to implement persistence.
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