Raft leader becomes follower if it can't communicate with a quorum.

If a leader can't communicate with a quorum, it can't process requests, but accepts them to the log. It may lead to filling the log with requests, which will not be processed in a reasonable time. So it's better for a leader to become follower and discard requests.
Fixed removing of nodes from cluster.

Before removing of node was supposed to be manual procedure. An administrator was supposed to remove the node from configuration mashine via control service and then shut down the node to disable its activity. Now a node always automatically stops any activity during removing of it. So nodes may be safely removed by any program.
Fixed automatic disabling of removed Raft leader.

When a leader removes itself from the cluster, it continues to be a leader, but doesn't count own votes during elections and replication. It's totaly safe but may be confusing. So now leader disables itself, after it commits self removing from the cluster.
Fix raft logs format.

All logs in the cocaine should be lowercase. Also new structured logs are used.
Fixed logs, part 2.
Little more encapsulation.

Now pointer to raft repository is a private field of the context.
Move Raft initialization to control service constructor.

Now raft control service is usual service and should be configured in config just like other services. And it initializes Raft.
If the service is not configured, Raft is disabled.
Merge pull request #94 from andrusha97/raft

Raft stuff