RFR: 7902989: jcstress: Add samples for high-level race conditions [v3]

[Michael Mirwaldt]

> I have created two more samples:
> 
> - one sample for read-modify-write-sequences
> - one sample for check-then-react-sequences

Michael Mirwaldt has updated the pull request incrementally with one additional commit since the last revision:

  changed copyright from 2020 to 2021

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Changes:
  - all: https://git.openjdk.java.net/jcstress/pull/88/files
  - new: https://git.openjdk.java.net/jcstress/pull/88/files/39c6b8d2..38b805b0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jcstress&pr=88&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jcstress&pr=88&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/88.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/88/head:pull/88

PR: https://git.openjdk.java.net/jcstress/pull/88