RFR: 8221359: Eliminate SATBMarkQueueSet::filter_thread_buffers

[Kim Barrett]

Please review this removal of the function used by G1 to clean up the
per-thread SATB queues when a young-only collection will have moved
young object references present in those queues.  This cleanup isn't
needed, since the same cleanup may be done before enqueuing a
completed queue buffer, and will also be done during the eventual
processing of the buffer contents.  Having this function called during
a pause just moves some work from the concurrent phase into a pause,
which isn't a good tradeoff.

Also renamed G1GCPhaseTimes::ExtRootScanSubPhases(Start|End) =>
ExtRootScanSubPhases(First|Last), to remove the ambiguity over whether
"end" is inclusive or exclusive.  (It was inclusive here.)

CR:
https://bugs.openjdk.java.net/browse/JDK-8221359

Webrev:
http://cr.openjdk.java.net/~kbarrett/8221359/open.00/

Testing:
mach5 tier1-3