RFR: 8140257: Add support for "gc service threads" to ConcurrentGCThread

[Derek White]

*Background*:
ConcurrentGCThread provides incomplete support for an initialization and 
termination protocol for GC threads, so missing parts are duplicated in 
almost all subclasses.

*Fix*:
Move duplicated run(), and stop() methods up from subclasses 
ConcurrentG1RefineThread, ConcurrentMarkThread, G1StringDedupThread, and 
G1YoungRemSetSamplingThread to ConcurrentGCThread, as well as declare 
virtual methods run_service() and stop_service.

Note that ConcurrentMarkSweepThread is the odd-ball subclass. It 
implements it's own termination protocol, it provides it's own run() and 
stop() methods, and does not use run_service() and stop_service().

*Bug*: https://bugs.openjdk.java.net/browse/JDK-8140257
*Webrev*: http://cr.openjdk.java.net/~drwhite/8140257/webrev.01/

*Tests*: jprt, Aurora "nightly" run (I think this is OK)
http://aurora.ru.oracle.com/faces/Batch.xhtml?batchName=1325690.VMSQE.adhoc.JPRT 

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