RFR (XS): 8205043: Make parallel reference processing default for G1

[Thomas Schatzl]

Hi all,

  can I have reviews for this split-off of JDK-8043575 to make parallel
reference processing in conjunction with dynamic number of thread
sizing default for G1?

We think that with recent changes to parallel reference processing it
is useful to do so, further nowadays we expect that most VMs are run
with more than one GC thread. So reference processing should benefit
from that as well by default.

Threads are by default automatically limited by the functionality
introduced with JDK-8043575 to avoid actually being slower than before
if using too many threads.

This is also the reason why we only suggest to make
ParallelRefProcEnabled default for G1: the thread sizing does not work
with other collectors. 

There is also a linked CSR for that change.

CR:
https://bugs.openjdk.java.net/browse/JDK-8205043
Webrev:
http://cr.openjdk.java.net/~tschatzl/8205043/webrev/
Testing:
new included test case

Thanks,
  Thomas