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

[William Kemper]

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

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Commit messages:
 - Factor computation of generational reserves into its own method
 - Combine old region surplus and deficit into a single balance field

Changes: https://git.openjdk.org/shenandoah/pull/410/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=410&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328626
  Stats: 133 lines in 5 files changed: 45 ins; 48 del; 40 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