RFR: Improve handling of promotion and old generation evacuation failures

[William Kemper]

There are two changes here:
 * A promotion failure will cause the old generation heuristic to request an old collection.
 * An evacuation failure in the old generation will trigger a full collection.

Minor change:
 * If an allocation failure has been detected, the control thread will not sleep at the bottom of its loop.

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Commit messages:
 - Reword assert message
 - Force FullGC when old generation evacuation fails
 - Schedule an old generation collection upon promotion failure
 - Only treat promotions as old gen allocations
 - Track allocation rate for each generation

Changes: https://git.openjdk.java.net/shenandoah/pull/81/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=81&range=00
  Stats: 80 lines in 6 files changed: 58 ins; 15 del; 7 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/81.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/81/head:pull/81

PR: https://git.openjdk.java.net/shenandoah/pull/81