RFR: Degenerate to young cycles for out of cycle allocation failures

[William Kemper]

### Summary
* Allocation failures outside of cycle degenerate to young collections (previously we ran a global cycle)
* Added new degen point "roots" because remembered set swap during init mark is _not_ idempotent
* There is also a change here to register old gen objects when they are allocated under a lock (this reverts a previous change)

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Commit messages:
 - Add new degen point for concurrent root mark
 - Register objects during allocation when heap is locked
 - Degenerate to young collection for allocation failure outside of cycle

Changes: https://git.openjdk.java.net/shenandoah/pull/91/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=91&range=00
  Stats: 75 lines in 10 files changed: 43 ins; 23 del; 9 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/91.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/91/head:pull/91

PR: https://git.openjdk.java.net/shenandoah/pull/91