BUG 5656 : Entity ownership candidates not removed consistently on leadership change

BUG 5656 : Entity ownership candidates not removed consistently on leadership change

This patch removes candidates for all downed members when entity ownership shard
leadership changes. This fixes a corner case where a leader/follower are both killed
simulaneously in a cluster which has greater than 3 nodes. In this case the old leader
does not have a chance to remove the killed follower. The new leader does know that
the follower is down so it can remove the candidates for all downed followers on
shard leadership change.

Change-Id: If28f5656e0daee40fb96a937dbd0a868b7d3645a
Signed-off-by: Moiz Raja <moraja@cisco.com>