RFR: 8328626: GenShen: Combine old generation surplus/deficit fields into a single balance field [v7]

[William Kemper]

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

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:

 - Merge remote-tracking branch 'shenandoah/master' into combine-surplus-deficit
 - Merge remote-tracking branch 'shenandoah/master' into combine-surplus-deficit
 - Merge remote-tracking branch 'shenandoah/master' into combine-surplus-deficit
 - Update comments
 - Consolidate region transfer logic, move generational specific code out of ShenandoahHeap
 - Use pass by reference for consistency with other methods
 - Fix zero build
 - Fix comments
 - Factor computation of generational reserves into its own method
 - Combine old region surplus and deficit into a single balance field

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Changes: https://git.openjdk.org/shenandoah/pull/410/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=06
  Stats: 277 lines in 10 files changed: 100 ins; 119 del; 58 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