RFR: Restrict generational mode to adaptive heuristic [v2]

[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.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:

 - Put verify option on separate line
 - Add generational modes to the test
 - Allow more memory for young generation
 - Update test to use shenandoah young gen pool when appropriate
 - Add generational mode test case
 - Don't make default mode explicit for consistency with other tests
 - Fix errors, improve test summaries
 - WIP: Bug fixes, add generational mode to more tests
 - WIP: Integrate generational mode with more tests
 - Restrict young generation adaptive heuristic
 - ... and 2 more: https://git.openjdk.org/shenandoah/compare/34d8f0de...28509ca0

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/233/files
  - new: https://git.openjdk.org/shenandoah/pull/233/files/50cc2f34..28509ca0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=233&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=233&range=00-01

  Stats: 11565 lines in 529 files changed: 6499 ins; 2392 del; 2674 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