RFR: Generation labeling for regions and allocation requests.

[Bernd Mathiske]

The original generational Shenandoah patch authored by Roman Kennke. 
The rest of the project will build upon this base.

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Commit messages:
 - Generation labeling for regions and allocation requests.

Changes: https://git.openjdk.java.net/shenandoah/pull/6/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=6&range=00
  Stats: 71 lines in 5 files changed: 61 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/6.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/6/head:pull/6

PR: https://git.openjdk.java.net/shenandoah/pull/6