RFR (S): 8201490: Improve concurrent mark keep alive closure performance

[Thomas Schatzl]

Hi,

  can I have reviews for this change that improves the performance of
the G1CMKeepAliveAndDrainClosure used for reference processing in some
(sometimes common) special case?

This particular change makes sure that the actual marking will not be
called for NULL referents in the keep-alive closure, and do not count
against the threshold that starts marking, so that marking will not
start without actual work.

Setup/teardown of G1 marking is very big.

CR:
https://bugs.openjdk.java.net/browse/JDK-8201490
Webrev:
http://cr.openjdk.java.net/~tschatzl/8201490/webrev
Testing:
hs-tier1-3

Thanks,
  Thomas