RFR (M): 8190426: Lazily initialize refinement threads with UseDynamicNumberOfGCThreads

[Thomas Schatzl]

Hi,

  can I have reviews for this change that makes refinement threads
behave the same as the other thread groups we have in G1?

I.e. with UseDynamicNumberOfGCThreads enabled (which is off by default)
they are created lazily.

This helps a little bit further with startup/footprint benchmarks (if
enabled).

CR:
https://bugs.openjdk.java.net/browse/JDK-8190426
Webrev:
http://cr.openjdk.java.net/~tschatzl/8190426/webrev/
Testing:
hs-tier1+2

Thanks,
  Thomas