MB-9725 Factor out a function to determine old map compatibility.

Change-Id: I42bbe42f736a60c28367133aed33b6d17435b36f
Reviewed-on: http://review.couchbase.org/31062
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>