RFR (S): 8226232: Move merge heap roots code out from G1RemSetScanState

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that moves code related to
the merge heap roots phase out of G1RemSetScanState? This imho
significantly increases readability of G1RemSetScanState.

More minor cleanups to improve the code are planned, but I think it is
good to take it in steps.

CR:
https://bugs.openjdk.java.net/browse/JDK-8226232
Webrev:
http://cr.openjdk.java.net/~tschatzl/8226232/webrev/
Testing:
hs-tier1-5

Thanks,
  Thomas