RFR: Track promotion rate into old generation

[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.

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Commit messages:
 - Merge branch 'shenandoah' into promotion-rate-heuristic
 - Remove unused bytes allocated tracking from heap
 - Logging to verify allocation rates for each generation
 - Merge branch 'shenandoah' into promotion-rate-heuristic
 - Track allocation rate for each generation

Changes: https://git.openjdk.java.net/shenandoah/pull/70/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=70&range=00
  Stats: 68 lines in 7 files changed: 47 ins; 14 del; 7 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