RFR: ThreadSnapshot: create objects in VM

[Alex Menkov]

The change removes upcalls to ThreadLock and ThreadSnapshot ctors, allocates and fills objects in the VM

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Commit messages:
 - jcheck
 - create objects in VM

Changes: https://git.openjdk.org/loom/pull/218/files
  Webrev: https://webrevs.openjdk.org/?repo=loom&pr=218&range=00
  Stats: 172 lines in 3 files changed: 120 ins; 36 del; 16 mod
  Patch: https://git.openjdk.org/loom/pull/218.diff
  Fetch: git fetch https://git.openjdk.org/loom.git pull/218/head:pull/218

PR: https://git.openjdk.org/loom/pull/218