[GEOS-6683] Ignore numberMatched per layer

This commit adds a value to the user- and rest interface for disabling
counting the numberMatched property. On large queries with very few
matches this can cause a significant speed boost.

NOTE: I currently have no test included in this commit. This is because I don't really know how to create a test for this change without changing the already supplied test data... When testing this on a live GeoServer instance however, the difference is notable. 
Curious why the type of the property is changed from BigInteger to String?