RFR: 8332095: GenShen: Move more generational mode members out of shHeap

[William Kemper]

The following members and methods in shHeap are only used in generational mode and can be moved out of common code:
* ShenandoahHeap::cancel_old_gc
* ShenandoahHeap::transfer_old_pointers_from_satb
* ShenandoahHeap::_is_aging_cycle

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Commit messages:
 - Move is_aging_cycle into genshen heap
 - Move 'cancel_old_gc' from shHeap into shOldGeneration
 - Inline forwarding method ShenandoahHeap::transfer_old_pointers_from_satb

Changes: https://git.openjdk.org/shenandoah/pull/431/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332095
  Stats: 101 lines in 12 files changed: 38 ins; 45 del; 18 mod
  Patch: https://git.openjdk.org/shenandoah/pull/431.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/431/head:pull/431

PR: https://git.openjdk.org/shenandoah/pull/431