Fix issue when AE leader differs from prior install snapshot leader

Fix issue when AE leader differs from prior install snapshot leader

When a leader snapshot install is in progress, a follower doesn't process
AppendEntries and merely returns a reply with its last term/index. However,
if an install snapshot is initiated by a leader and there is more than one
chunk to send, it's possible for a leader change to occur prior to completing
sending all the chunks. When this happens, the new leader will begin sending
AppendEntries but the follower won't process them and make progress. We need
to clear the follower's snapshot state if the AppendEntries leaderId doesn't
match the prior install snapshot leaderId.

Change-Id: I4051bd064b6a20f4bcfe38b50656488fcb09274e
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>