Implement pauseLeader timeout for leadership transfer

Implement pauseLeader timeout for leadership transfer

Added an abstract TimedRunnable class that implements a Runnable operation
with a timer such that if the run method isn't invoked within a timeout
period, the operation is cancelled. The
RaftActorLeadershipTransferCohort passes a TimedRunnable instance to
pauseLeader to abort the transfer if pauseLeader doesn't complete within
an election timeout period.

Change-Id: I773605117dc4e310f3ee5025c0131b9f1447c746
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>