RFR (): 8140255: Move the management of G1YoungRemSetSamplingThread from G1ConcurrentRefine

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that moves the ownership of the
young gen sampling thread from G1ConcurrentRefine to G1CollectedHeap,
in the process removing a few comments like "// The RS sampling thread
has nothing to do with refinement, but is here for now.".

CR:
https://bugs.openjdk.java.net/browse/JDK-8140255
Webrev:
http://cr.openjdk.java.net/~tschatzl/8140255/webrev/
Testing:
hs-tier1, hs-tier2, local testing

Thanks,
  Thomas