RFR [S][2/7] 8197570: Make rules for choosing collection set candidates more explicit

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that makes the rule (occupancy)
for choosing collection set candidates more explicit, i.e. factor it
into a single method. This is needed for selecting the regions we want
to rebuild the remembered sets concurrently (JDK-8180415) more
straightforward.

It also adds a method to allow iteration over the regions in the
collection set - I did not see that worth an extra webrev.

CR:
https://bugs.openjdk.java.net/browse/JDK-8197570
Webrev:
http://cr.openjdk.java.net/~tschatzl/8197570/webrev/
Testing:
hs-tier 1-5, mostly in conjunction with related changes

Thanks,
  Thomas