RFR: Do old satb during interrupting young

[Kelvin Nilsen]

This patch includes bug fixes discovered during integration and testing of concurrent remembered set scanning.  The bug is that processing of SATB barrier by young-gen collections that preempt a concurrent old-gen concurrent marking effort was ignoring pointers to old-gen.  It needs to pass these pointers along to the old-gen concurrent mark effort.

Since the bug manifests even outside concurrent remembered set scanning, we propose to upstream it now.

The patch has been tested with jtreg:tier1_gc_shenandoah with no regressions.

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Commit messages:
 - Stronger encapsulation of bootstrap gc concept
 - Remove duplicate asserts, init global generation for bootstrap (rather than old + young, which visits free regions twice).
 - Remove completed TODO comment
 - Make old gc bootstrap flag immutable
 - Preserve access to old gen mark queues for young collects

Changes: https://git.openjdk.java.net/shenandoah/pull/51/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=51&range=00
  Stats: 47 lines in 7 files changed: 20 ins; 12 del; 15 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/51.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/51/head:pull/51

PR: https://git.openjdk.java.net/shenandoah/pull/51