RFR: Borrow from old gen [v3]

[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.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Fixup mistakes from previous commit

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/110/files
  - new: https://git.openjdk.java.net/shenandoah/pull/110/files/4129539b..56a27bef

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=110&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=110&range=01-02

  Stats: 15 lines in 2 files changed: 0 ins; 15 del; 0 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