RFR: Track promotion rate into old generation [v2]

[William Kemper]

> The member `_bytes_allocated_since_last_gc` has been moved into the `ShenandoahGeneration` abstraction and is updated accordingly. Evacuations into the old generation (i.e., promotion) are counted as allocations against the old generation. Promotion of entire regions is not recorded in the promotion rate.

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

 - Only register successful old evacuations
 - Only treat promotions as old gen allocations

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/70/files
  - new: https://git.openjdk.java.net/shenandoah/pull/70/files/9f4d5d20..588c5203

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=70&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=70&range=00-01

  Stats: 71 lines in 3 files changed: 18 ins; 45 del; 8 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/70.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/70/head:pull/70

PR: https://git.openjdk.java.net/shenandoah/pull/70