MB-8495 Increase docs_total estimate when docs_left increases.

It does not fix the root cause of the referenced bug. There we just
happened to see CHECKPOINT_START/CHECKPOINT_END messages. But since
it still seems to be possible to hit the same issue when there're
mutations going on during rebalance, I decided to make this fix.

Change-Id: Ic6b3ad28806ce1f353ca98882372b75e7930caf4
Reviewed-on: http://review.couchbase.org/28275
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>