RFR (S): 8227089: Add timing information for merging humongous remembered sets

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that adds timing information for
the part of the merge heap roots phase that dumps eager reclaim
candidates onto the card table?

I am aware that the eager reclaim phase is actually single threaded -
there is an extra CR for fixing this (JDK-8141637). At least with
recent change that effort is hidden by the other work in this phase;
but until now we did not know that there may be an issue.

CR:
https://bugs.openjdk.java.net/browse/JDK-8227089
Webrev:
http://cr.openjdk.java.net/~tschatzl/8227089/webrev
Testing:
hs-tier1-5

Thanks,
  Thomas