BUG 2371 : Leader should reset it's snapshot tracking when follower is restarted

This patch adds a new protocol to InstallSnapshot. It the InstallSnapshotReply returns
a failure and the chunkIndex is -1 then the Leader will reset the FollowerSnapshot so
that when the next heartbeat occurs the Leader would start sending chunks from the beginning.

Change-Id: I0d5f0a4230209856ecf9bcef46220ae348f52b5d
Signed-off-by: Moiz Raja <moraja@cisco.com>