MB-11919 Log sequence number difference in indexing

Due to MB-11589, we are suspecting that view indexer has to do more
work. Added a message to log how many extra mutations we are getting
in indexing during rebalance.

Change-Id: Ibd6fb2e43d384ecfe98174fce5f296159aaefe42
Reviewed-on: http://review.couchbase.org/40617
Tested-by: Nimish Gupta <nimish@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>