RFR: 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field [v3]

[William Kemper]

> This is a follow up to https://github.com/openjdk/shenandoah/pull/406.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Consolidate region transfer logic, move generational specific code out of ShenandoahHeap
 - Use pass by reference for consistency with other methods

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/410/files
  - new: https://git.openjdk.org/shenandoah/pull/410/files/8ebe8b8c..d75de101

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=01-02

  Stats: 156 lines in 7 files changed: 52 ins; 75 del; 29 mod
  Patch: https://git.openjdk.org/shenandoah/pull/410.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/410/head:pull/410

PR: https://git.openjdk.org/shenandoah/pull/410