Remove unused Raft actor states.

Actor state is only used to determine if the actor is a leader. Now states are removed and atomic bool flag is used for this purpose. By the way method is_leader() has became thread-safe.
Split Raft service into two services.

Now there is raft control service, which is supposed to be used by a system administrator, and raft node service, which is used by Raft internally.
Add some methods to Raft control service.

Now it has a method to get configuration of all state machines and a method to retrieve current leader of a state machine.