Merge SATBMarkQueueFilter into SATBMarkQueueSet

The classes SATBMarkQueueFilter and G1SATBMarkQueueFilter were recently added (JDK-8208611). Since then SATBMarkQueueSet has been refactored to have subclasses to allow different GC implementions of the thread-specific queue access (JDK-8209118). This subclass provides a better place for the filtering support, and we should merge the Filter class into the QSet class.