RFR: Age cycle periodically

[Kelvin Nilsen]

Add option to allow object ages to increment periodically instead of on every young cycle.

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Commit messages:
 - Turn off aging cycle on a safepoint and remove assert from vmop_entry_final_updaterefs
 - Do not use GC State flag for is_aging_cycle
 - Implement period for increasing age

Changes: https://git.openjdk.java.net/shenandoah/pull/80/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=80&range=00
  Stats: 38 lines in 8 files changed: 33 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/80.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/80/head:pull/80

PR: https://git.openjdk.java.net/shenandoah/pull/80