[BUGFIX] 10638 WFS GetFeature may be incomplete if layer reports estimated feature count

layer->featureCount() sometimes returns an estimated feature count.
In this case, the WFS server returns an incomplete reply (if called without
 maxfeatures).

To make QgsWFSServer::getFeature a bit more robust, it's better to do not take
 account of layer->featureCount() and use
 while ( fit.nextFeature( feature ) && ( featureCounter < maxFeat || maxFeatures == -1 ) )