RFR: JDK-8303742: CompletableFuture.orTimeout leaks if the future completes exceptionally [v4]

[Viktor Klang]

> Addresses the situation where exceptional completion of `orTimeout`:ed CompletableFutures wouldn't cancel the timeout task which could lead to memory leaks if done frequently enough with long enough timeout durations.
> 
> Fix discussed with @DougLea

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Adding a test for completeOnTimeout cancelling tasks when completeExceptionally

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13059/files
  - new: https://git.openjdk.org/jdk/pull/13059/files/05619280..e2c1c366

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13059&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13059&range=02-03

  Stats: 13 lines in 1 file changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13059.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13059/head:pull/13059

PR: https://git.openjdk.org/jdk/pull/13059