RFR: 8297149: REDO JDK-8296889: Race condition when cancelling a request [v2]

[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.

Daniel Fuchs has updated the pull request incrementally with one additional commit since the last revision:

  Fixed onSubscribed() doc comment and override access qualifiers (protected vs public)

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11193/files
  - new: https://git.openjdk.org/jdk/pull/11193/files/fd527d54..60bbce6d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11193&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11193&range=00-01

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 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