RFR: CODETOOLS-7902950: jcstress: Offload VM start/finish/output tasks to separate workers

[Aleksey Shipilev]

The TestExecutor loop is busy driving the run, and the blocking code that starts/finishes VMs should be offloaded to separate workers. Additionally, InputStreamCollectors that currently run as separate threads, can use the threadpool.

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Commit messages:
 - Pump VMs

Changes: https://git.openjdk.java.net/jcstress/pull/58/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=58&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902950
  Stats: 117 lines in 3 files changed: 74 ins; 26 del; 17 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/58.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/58/head:pull/58

PR: https://git.openjdk.java.net/jcstress/pull/58