RFR: 8331609: GenShen: Refactor generational mode allocations [v2]

[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 one additional commit since the last revision:

  Fix zero build, warnings in test

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/427/files
  - new: https://git.openjdk.org/shenandoah/pull/427/files/b6860298..b2163411

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=427&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=427&range=00-01

  Stats: 17 lines in 1 file changed: 2 ins; 0 del; 15 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