MB-11608 Don't require halt() to complete outstanding IO.

This was the default in R14.

Change-Id: I837b2a22a6f78d8c73ed8ac8d529bb98784f1596
Reviewed-on: http://review.couchbase.org/39227
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>