Pick up delta recovery map only if it's trivially matching.

Change-Id: I4c0136e7ea3b1dee397c9d7f0b613469e804e04e
Reviewed-on: http://review.couchbase.org/38282
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>