RFR: 7903890: jcstress: Rewrite/amend Singleton and Lazy primitives samples

[Aleksey Shipilev]

It is obvious we do not have a good narrative around these, so let's add some.

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Commit messages:
 - Lazy + Singleton examples

Changes: https://git.openjdk.org/jcstress/pull/155/files
  Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=155&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903890
  Stats: 3045 lines in 37 files changed: 2243 ins; 802 del; 0 mod
  Patch: https://git.openjdk.org/jcstress/pull/155.diff
  Fetch: git fetch https://git.openjdk.org/jcstress.git pull/155/head:pull/155

PR: https://git.openjdk.org/jcstress/pull/155