RFR: Fix full gc with class unloading

[Kelvin Nilsen]

This pull request fixes incompatibilities between Full GC and class unloading.  In particular, when degenerated GC upgrades to Full GC, it is important to change the generation to GLOBAL.  Otherwise, certain assertions related to class unloading fail.

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Commit messages:
 - Merge branch 'shenandoah' into fix-class-unloading-with-full-gc
 - Respond to reviewer feedback
 - Fix up state diagram
 - Update state diagram to incorporate full gc
 - Fix class unloading interactions with full gc

Changes: https://git.openjdk.java.net/shenandoah/pull/67/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=67&range=00
  Stats: 60 lines in 4 files changed: 31 ins; 5 del; 24 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/67.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/67/head:pull/67

PR: https://git.openjdk.java.net/shenandoah/pull/67