RFR: Add generational full gc support

[Kelvin Nilsen]

In situations when Generational Shenandoah triggers full-gc, we now allow full gc to run.  The implementation of full gc preserves the distinction between young and old memory regions because premature promotion of young objects into old regions may result in inefficient subsequent old-gc efforts.

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Commit messages:
 - Fix white space
 - Merge branch 'shenandoah' into full-gc-with-generations
 - Preserve and restore original element count for card table barrier
 - Address reviewer feedback
 - Merge branch 'shenandoah' into full-gc-with-generations
 - Respond to reviewer feedback
 - Fix two errors in Full GC support
 - Instrumentation to help debug
 - Fix some bugs in Full GC
 - Fixup verification at mark and add instrumentation
 - ... and 6 more: https://git.openjdk.java.net/shenandoah/compare/9f0f23b7...c93f9599

Changes: https://git.openjdk.java.net/shenandoah/pull/61/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=61&range=00
  Stats: 857 lines in 22 files changed: 656 ins; 155 del; 46 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/61.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/61/head:pull/61

PR: https://git.openjdk.java.net/shenandoah/pull/61