RFR: 8297149: REDO JDK-8296889: Race condition when cancelling a request

[Daniel Fuchs]

Please find here a re-do fix for the race condition while cancelling request.
The previous fix failed because it registered the subscriber too late (after having called userSubsciber.onSubscribe()), which opened a window for the call to unregister to occur before the call to register.
This is fixed in this new iteration.

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Commit messages:
 - Fix issue list
 - 8297149

Changes: https://git.openjdk.org/jdk/pull/11193/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11193&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297149
  Stats: 44 lines in 5 files changed: 31 ins; 6 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/11193.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11193/head:pull/11193

PR: https://git.openjdk.org/jdk/pull/11193