RFR: ThreadSnapshot: redesign and cleanup [v3]

[Alex Menkov]

> ThreadSnapshot: redesign and cleanup

Alex Menkov has updated the pull request incrementally with two additional commits since the last revision:

 - jcheck
 - update

-------------

Changes:
  - all: https://git.openjdk.org/loom/pull/220/files
  - new: https://git.openjdk.org/loom/pull/220/files/5269e798..692bdb23

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=loom&pr=220&range=02
 - incr: https://webrevs.openjdk.org/?repo=loom&pr=220&range=01-02

  Stats: 120 lines in 3 files changed: 43 ins; 25 del; 52 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