RFR: Refactor budgeting to make the logic cleaner [v6]

[Kelvin Nilsen]

> This commit consolidates computations related to budgeting of reserves for old-gen evacuation and old-gen promotion within shenandoahGeneration::prepare_regions_and_collection_set().  Previously, these computations had been scattered between several functional units, including shenandoahHeuristics::choose_collection_set() and shenandoahOldHeuristics::prime_collection_set().  A previous pull request brought some of the redundant functionality into prepare_regions_and_collection_set().  This pull request removes the redundant computations from the other locations.

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

  Make helper methods private

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/148/files
  - new: https://git.openjdk.org/shenandoah/pull/148/files/6c42841a..93a16843

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=148&range=05
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=148&range=04-05

  Stats: 22 lines in 1 file changed: 12 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/148.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/148/head:pull/148

PR: https://git.openjdk.org/shenandoah/pull/148