RFR (S): 8154528: Reclaim regions emptied by marking in Remark pause

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that moves the space reclamation
of empty regions into the Remark pause, which makes these regions
available much sooner than before, with the obvious benefits of doing
so.

The Cleanup pause now only contains updating the remembered set states
after rebuilding remembered sets and determining the collection set
candidates. In the future we might be able to make those concurrent and
drop the Cleanup pause completely.

There is not much too say here, after all the recent refactorings this
patch is almost trivial :)

>From a timing POV this adds a few ms to the Remark pause in my
measurements, mostly from the code root purging moved here. I think
there is enough opportunity in the remark pause to decrease its
duration in the future that outweighs the mentioned benefits.

Thanks go to everyone reviewing so far, particularly StefanJ and
Sangheon!

Depends on JDK-8178105.

CR:
https://bugs.openjdk.java.net/browse/JDK-8154528
Webrev:
http://cr.openjdk.java.net/~tschatzl/8154528/webrev/
Testing:
hs-tier 1-5

Thanks,
  Thomas