MB-8363 Set numThreads minimum value to 1.

Change-Id: Ic273744c1076501031af2ba0a9bb6d43368ff23d
Reviewed-on: http://review.couchbase.org/26737
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>