RFR: Add 'generational' as value choice for JVM option 'ShenandoahGCMode'. [v3]

[Bernd Mathiske]

> All upcoming GenShen code will be guarded by this JVM option. If generational mode is not set, Shenandoah will function in its original single-generational form, without changes or degradations.

Bernd Mathiske has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Add 'generational' as value choice for JVM option 'ShenandoahGCMode'.

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/4/files
  - new: https://git.openjdk.java.net/shenandoah/pull/4/files/39b9ee06..60ba9d90

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=4&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=4&range=01-02

  Stats: 6 lines in 4 files changed: 0 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/4.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/4/head:pull/4

PR: https://git.openjdk.java.net/shenandoah/pull/4