RFR: Borrow from old gen

[Kelvin Nilsen]

This commit represents multiple months of incremental performance improvements to allow generational shenandoah to run more efficiently, especially with larger heap sizes and high memory utilization.

Specific improvements are described in individual commit log messages.

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

Commit messages:
 - Whitespace
 - Fix whitespace and other cosmetics
 - Remove debugging instrumentation
 - Simplify crossing map implementation
 - Restore young-gen->used() and capacities at end of degenerated GC
 - Claim credit for Old and Young GC following GLOBAL/Full GC
 - Fix pad accounting for plab alignment
 - Three changes:
 - Fix argument in instrumentation calls
 - Fix bug in ShenandoahCardCluster::coalesce_objects()
 - ... and 6 more: https://git.openjdk.java.net/shenandoah/compare/b233fe7c...dc8a6654

Changes: https://git.openjdk.java.net/shenandoah/pull/110/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=110&range=00
  Stats: 1473 lines in 33 files changed: 1264 ins; 79 del; 130 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/110.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/110/head:pull/110

PR: https://git.openjdk.java.net/shenandoah/pull/110