RFR (M): 8213229: Treat StringTable as weak in G1 young collections

[Thomas Schatzl]

Hi all,

  can I have reviews that treat the string table as weak during young
gc (too)?

The change itself is pretty straightforward, i.e. not processing the
string table as roots (if ClassUnloading is true - this seems to be the
commonly used flag to turn this off), and at the end in some parallel
phase purge the string table itself.

Unfortunately this incurred some changes to the parallel cleaning tasks
to allow doing this parallel phase in conjunction with string
deduplication to avoid the additional worker thread startup, and share
some code, which rippled through the code a bit.

However this also makes it a bit easier imho to join other parallel
phases like the weak handle processing during the post evacuation phase
into this "partial cleanup" in follow-ups.

CR:
https://bugs.openjdk.java.net/browse/JDK-8213229
Webrev:
http://cr.openjdk.java.net/~tschatzl/8213229/webrev/
Testing:
hs-tier1-5, gc/shenandoah for shenandoah

Thanks,
  Thomas