RFR (S): 8076463: Add logging for the preserve CM referents task

[Thomas Schatzl]

Hi all,

  can I have reviews for the following change that adds logging for the
Preserve CM referents task that has previously been summarized under
the "Other" category?

It sometimes seems that this part of the GC takes a long time, so these
measurements at least increase awareness of any problem.

I also took the opportunity to move that code related to that phase
into a new method. This will slightly ease any further work in this
area.

CR:
https://bugs.openjdk.java.net/browse/JDK-8076463
Webrev:
http://cr.openjdk.java.net/~tschatzl/8076463/webrev/
Testing:
jprt, unit test

Thanks,
  Thomas