RFR: CODETOOLS-7902964: jcstress: Introduce short-stride checkpointing for more frequent collisions

[Aleksey Shipilev]

Currently, the harness uses larger strides to run and consume the results of the test. Unfortunately, when actor threads are unbalanced (e.g. run asymmetric workloads), there is a single collision per entire stride. Having shorter strides would increase collision ratios, but would create much more overhead for creating/consuming benchmark results and state.

We can do long "epoch" with several shorter "strides" and spin-loop checkpointing between strides, so that threads can catch up more often.

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Commit messages:
 - Round to 256
 - await
 - Fixes
 - Rename to checkpoints
 - Progress
 - Simplify strides
 - Update to 256
 - Epoch tombstones

Changes: https://git.openjdk.java.net/jcstress/pull/69/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=69&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902964
  Stats: 134 lines in 7 files changed: 16 ins; 53 del; 65 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/69/head:pull/69

PR: https://git.openjdk.java.net/jcstress/pull/69