[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. 
Ok. Another way to do this would just be to use a null value to represent unknown and then during the encoding recognize that and emit the string unknown. It would be a bit cleaner than making the field a string and constantly converting to a number to actually do anything with it.