RFR: 8331609: GenShen: Refactor generational mode allocations [v4]

[William Kemper]

> * The code for resizing and retrying young allocation requests according to what's available in the young generation has been removed. This code became redundant after the free set was rewritten and began providing the same functionality.
> * An unused field and its attendant methods were removed from `ShenandoahThreadLocalData`.
> * The code for managing `plab` expenditures and configuring other plab behavior has been moved from `ShenandoahHeap` into `ShenandoahOldGeneration`.

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

 - Improve comments, remove too strong assert
 - Initialize _evacuation_reserve member
 - Modify test fixture to avoid accessing ShenandoahHeap before we know if UseShenandoahGC is set

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/427/files
  - new: https://git.openjdk.org/shenandoah/pull/427/files/a2c52db1..4320206b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=427&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=427&range=02-03

  Stats: 81 lines in 3 files changed: 23 ins; 23 del; 35 mod
  Patch: https://git.openjdk.org/shenandoah/pull/427.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/427/head:pull/427

PR: https://git.openjdk.org/shenandoah/pull/427