RFR: 8138920: Refactor the sampling thread from ConcurrentG1RefineThread

[Derek White]

Refactor and cleanup the G1 concurrent thread classes:
  - Pull out a sampling thread class (now ConcurrentG1SampleThread) from 
ConcurrentG1RefineThread.
  - Factor out an abstract base class ConcurrentG1ServiceThread that is 
used by:
     - ConcurrentG1RefineThread
     - ConcurrentG1SampleThread
     - ConcurrentMarkThread
  - Made the handling of the "primary" refinement thread more explicit.
  - Updated obsolete and confusing comments

This is tech debt that also will allow disabling concurrent refinement 
(if desired) and also fixes a P4 bug.
Patch started by Thomas and improved and/or mangled myself.

RFE: JDK-8138920 <https://bugs.openjdk.java.net/browse/JDK-8138920> 
Refactor the sampling thread from ConcurrentG1RefineThread

Webrev: http://cr.openjdk.java.net/~drwhite/8138920/webrev.01/

Testing: jprt

Perf testing: in progress...


Thanks!

  - Derek



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