RFR: 8231153: Improve concurrent refinement statistics

[Kim Barrett]

Please review this change to collection additional statistics about
the generation and processing of dirty cards.  These are needed for
future improvements to the control of the concurrent refinement
threads (JDK-8137022).

This change adds tracking and prediction of the rate at which
concurrent refinement threads process cards.  It also adds tracking
and prediction of the rate at which mutator threads generate cards.

As part of that, there is now tracking of total cards processed by the
concurrent refinement threads and the mutator threads.  This is now
used by G1RemsetSummary instead of the buffer counts previously reported.

CR:
https://bugs.openjdk.java.net/browse/JDK-8231153

Webrev:
https://cr.openjdk.java.net/~kbarrett/8231153/open.00/

Testing:
mach5 tier1-5
some local by-hand testing to look at the rate tracking.