BUG 2817 - Basic implementation of RemoveServer in the Raft code

When a RemoveServer is received it may ask for the removal of
a the current leader or one of the followers. As a first pass
we do not support removal of the current leader. To correctly
implement removal of the leader we would have to implement
leader transition which I intend to build in a future patch.

When a follower is removed the server configuration is changed
immediately on the leader and the new configuration persisted
to the journal. When other followers receive the removed
journal entry they would also remove the server from their
configuration, this is the same as what was done for the
AddServer implementation.

As soon as then new configuration is persisted we respond with
success to the caller. This is the same as for AddServer.

When the ServerConfiguration is complete we send a ServerRemoved
message to the follower which has been removed.

Change-Id: I2b85d82cbeef13cca830e3cc212aebbbcd95c818
Signed-off-by: Moiz Raja <moraja@cisco.com>