RFR: ThreadSnapshot: redesign and cleanup

[Alex Menkov]

ThreadSnapshot: redesign and cleanup

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Commit messages:
 - fix

Changes: https://git.openjdk.org/loom/pull/220/files
  Webrev: https://webrevs.openjdk.org/?repo=loom&pr=220&range=00
  Stats: 213 lines in 7 files changed: 82 ins; 56 del; 75 mod
  Patch: https://git.openjdk.org/loom/pull/220.diff
  Fetch: git fetch https://git.openjdk.org/loom.git pull/220/head:pull/220

PR: https://git.openjdk.org/loom/pull/220