10 RFR: 8169517: WhiteBox should provide concurrent GC phase control

[Kim Barrett]

StefanK and Per had some offline comments too.

* Added WhiteBox.getConcurrentGCPhases().

* Renamed TestConcurrentPhaseControlSupport to
ConcurrentPhaseControlUtil. Simplified usage, eliminating subclassing
and associated function overrides in favor of constructor arguments.

* Moved TestConcurrentPhaseControlBasics and
TestConcurrentPhaseControlG1 to test/gc/whitebox/concurrent_phase_control.

* Added match test between expected phases and actual. This tests the
new getConcurrentGCPhases() function.

* Added ConcurrentGCPhaseManager::Stack, reifying the previously
implicit stack. This simplifies the manager API and G1's usage.

* Added REMARK manager phase for G1, mostly as an example and to make
that section a little more clear.

New webrevs:
full:
http://cr.openjdk.java.net/~kbarrett/8169517/hs.01/
http://cr.openjdk.java.net/~kbarrett/8169517/hotspot.01/

incremental:
http://cr.openjdk.java.net/~kbarrett/8169517/hs.01.inc/
http://cr.openjdk.java.net/~kbarrett/8169517/hotspot.01.inc/