Add method to retrieve status of Raft node.

Now it's possible to discover if a node is follower, candidate, leader, or it even hasn't joined a cluster. This information is usefull to monitor cluster state.
Typo.
Add proper includes.

Classes in forwards.hpp use boost::optional and boost::variant, so corresponding headers should be included.
Don't run reactor from two threads.

Each actor creates own thread and runs its reactor there. So passing one reactor to two actors is a bad idea.
Merge pull request #82 from andrusha97/raft-monitorings

Raft monitoring