RFR: Restrict generational mode to adaptive heuristic

[William Kemper]

This is a small change to Shenandoah, but it prevents us from running the tests with the generational mode enabled via a jtreg vm option. This requires us to properly integrate the generational mode with the jtreg tests. There are many test files changed, but the `generational` mode is now handled in the same fashion as other modes (`iu`, `passive`). There are two additional advantages to these changes:
* We no longer need to run the test suite twice (once in the default mode and again with `-XX:ShenandoahGCMode=generational`).
* Expanded test coverage, as some tests would spawn their own VMs with permutations of mode and heuristic options that ignored the generational mode.

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Commit messages:
 - Merge branch 'shenandoah-master' into restrict-generational-heuristic
 - WIP: Bug fixes, add generational mode to more tests
 - WIP: Integrate generational mode with more tests
 - Restrict young generation adaptive heuristic
 - WIP: Properly integrate generational mode testing (compiler tests)
 - WIP: Properly integrate generational mode testing
 - Do not visit gc threads during shutdown

Changes: https://git.openjdk.org/shenandoah/pull/233/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=233&range=00
  Stats: 1720 lines in 82 files changed: 1345 ins; 312 del; 63 mod
  Patch: https://git.openjdk.org/shenandoah/pull/233.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/233/head:pull/233

PR: https://git.openjdk.org/shenandoah/pull/233