RFR: 8295319: pending_cards_at_gc_start doesn't include cards in thread buffers

[Kim Barrett]

Please review this change to G1 to include the per-thread buffers in the
number of pending cards at the start of a young GC.

DCQS::concatenate_logs has been renamed to concatenate_logs_and_stats, and now
also merges the per-thread refinement stats during the thread walk to flush
buffers.  That replaces the separate thread walk to merge and record these
stats earlier in the GC.  The merged stats and related info don't seem to be
needed until after the buffer flushing.

Also, when abandoning dirty card buffers and stats because of a full GC, fixed
to also abandon an buffers in the paused buffers lists.

Testing:
mach5 tier1-3
performance testing found no significant changes, as expected.

-------------

Commit messages:
 - move refinement stats collecting and recording

Changes: https://git.openjdk.org/jdk/pull/10914/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10914&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295319
  Stats: 103 lines in 7 files changed: 36 ins; 38 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/10914.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10914/head:pull/10914

PR: https://git.openjdk.org/jdk/pull/10914