RFR: Refactor budgeting to make the logic cleaner

[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.

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Commit messages:
 - Refactor budgeting to make the logic cleaner

Changes: https://git.openjdk.org/shenandoah/pull/148/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=148&range=00
  Stats: 216 lines in 4 files changed: 12 ins; 196 del; 8 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