RFR: Generation affiliation transitions for heap regions.

[Bernd Mathiske]

Preparatory renaming generation->affiliation since we will want to have a class called ShenandoahGeneration that is not an enum. The next PR will build on this. It will be a big refactoring that pulls code out of the ShenandoahHeap class and puts it into generation-specific classes that are subclasses of ShenandoahGeneration.

The Affiliation declaration still sits in AllocRequest and not in ShenandoahHeapRegion or ShenandoahGeneration for the same reason as before: it is hard to get anything to compile in other combinations of what goes where. I tried.

The card table update code in the set_affiliation() will be revised and augmented later, but seems OK as a first step.

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Commit messages:
 - Generation affiliation transitions for heap regions.

Changes: https://git.openjdk.java.net/shenandoah/pull/7/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=7&range=00
  Stats: 178 lines in 11 files changed: 138 ins; 5 del; 35 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/7.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/7/head:pull/7

PR: https://git.openjdk.java.net/shenandoah/pull/7