RFR (M): JDK-8035401: Fix visibility of G1ParScanThreadState members

[Thomas Schatzl]

Hi all,

  can I have reviews for this change? After moving G1ParScanThreadState,
this change cleans up visibility, making a whole lot of stuff private.

CR:
https://bugs.openjdk.java.net/browse/JDK-8035401

Webrev:
http://cr.openjdk.java.net/~tschatzl/8035401/webrev/

Testing:
perf testing indicated no changes, jprt

Thanks,
  Thomas