RFR: 8344797: GenShen: Update and rename confusing method

[William Kemper]

A few riders here:
* [Remove vestigial debugging assertions](https://github.com/openjdk/shenandoah/commit/9062113458bd1909409967abb743e11982f351ab)
* [Log cancellation time at debug level](https://github.com/openjdk/shenandoah/commit/e3dd23efcb179bc61565d25f6b46f3690a50caa4)
* [Use try_set instead of set](https://github.com/openjdk/shenandoah/pull/550/commits/49cb66a0fa7d0d660f5b986eea9d71035c157310)

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Commit messages:
 - Use try_set instead of set
 - Update and rename confusing method to match intended usage
 - Log cancellation time at debug level
 - Remove vestigial debugging assertions

Changes: https://git.openjdk.org/shenandoah/pull/550/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=550&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344797
  Stats: 23 lines in 8 files changed: 3 ins; 4 del; 16 mod
  Patch: https://git.openjdk.org/shenandoah/pull/550.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/550/head:pull/550

PR: https://git.openjdk.org/shenandoah/pull/550