RFR: Align plabs on card boundaries

[Kelvin Nilsen]

If plabs are not aligned on remembered set card boundaries, then we need synchronization locks to register the promotion objects allocated by each thread.  We do not currently have these synchronization locks and do not want to add them.  Instead, we are enforcing that all of the objects that reside within a particular remembered set card are allocated by the same thread.

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Commit messages:
 - Fix whitespace
 - Respond to reviewer feedback
 - Debug plab alignment and remove instrumentation
 - Align PLAB allocations on remembered set card boundaries

Changes: https://git.openjdk.java.net/shenandoah/pull/95/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=95&range=00
  Stats: 95 lines in 4 files changed: 87 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/95.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/95/head:pull/95

PR: https://git.openjdk.java.net/shenandoah/pull/95