RFR: Enable skip update references

[William Kemper]

This feature of Shenandoah was disabled for generational mode for the reasons described in [PR 50](https://github.com/openjdk/shenandoah/pull/50). Old objects which are no longer reachable and _not_ included in the collection set are filled in at the end of final marking for the old generation so we no longer require the update references phase to perform this task.

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Commit messages:
 - Merge branch 'shenandoah' into test-skip-update-references
 - Re-enable immediate collection of 100% garbage regions

Changes: https://git.openjdk.java.net/shenandoah/pull/54/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=54&range=00
  Stats: 3 lines in 2 files changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/54.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/54/head:pull/54

PR: https://git.openjdk.java.net/shenandoah/pull/54