Refactor SATBMarkQueue filter configuration

SATBMarkQueue::should_enqueue_buffer compares the fullness of the post-filtered buffer with G1SATBBufferEnqueueingThresholdPercent. This reference to a G1-specific configuration variable needs to be changed in order to allow the SATBMarkQueue code to be used by other collectors. The configured value could be stored in the SATBQSet, initialized by GC-specific code.