[jdk11u-dev] RFR: 8323670: A few client tests intermittently throw ConcurrentModificationException

[Martin Doerr]

Backport of [JDK-8323670](https://bugs.openjdk.org/browse/JDK-8323670). Applies cleanly except Copyright year update.

-------------

Commit messages:
 - Backport 70e7cdcb9ecedd455cf701b5c7ad05286ac0067d

Changes: https://git.openjdk.org/jdk11u-dev/pull/2766/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323670
  Stats: 48 lines in 1 file changed: 6 ins; 4 del; 38 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2766.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2766/head:pull/2766

PR: https://git.openjdk.org/jdk11u-dev/pull/2766