fix #1486 Prevent usingWhen terminal & cancel callbacks to both apply

[simonbasle]

This commit prevents asyncComplete/asyncError terminal callbacks to
apply in addition to asyncCancel, in case one signal is still received
after the other (eg. a cancellation is performed asynchronously,
resulting in the signal arriving AFTER the onError/onComplete signals).

[simonbasle]

cc @mp911de can you look at the added tests see if they correctly cover your case?

[codecov-io]

Codecov Report

Merging #1487 into master will increase coverage by 0.03%.
The diff coverage is 96.29%.

@@             Coverage Diff              @@
##             master    #1487      +/-   ##
============================================
+ Coverage     84.14%   84.17%   +0.03%     
  Complexity     3898     3898              
============================================
  Files           359      359              
  Lines         29831    29835       +4     
  Branches       5531     5534       +3     
============================================
+ Hits          25102    25115      +13     
+ Misses         3094     3091       -3     
+ Partials       1635     1629       -6

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/reactor/core/publisher/FluxUsingWhen.java	96.48% <96.29%> (+0.07%)	8 <0> (ø)	⬇️
.../java/reactor/core/publisher/UnicastProcessor.java	87.15% <0%> (-1.12%)	68% <0%> (-1%)
.../java/reactor/core/publisher/BlockingIterable.java	78.12% <0%> (-1.05%)	7% <0%> (ø)
...java/reactor/core/publisher/FluxSwitchOnFirst.java	74.32% <0%> (-0.68%)	3% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxCreate.java	86.88% <0%> (+0.24%)	8% <0%> (ø)	⬇️
...ain/java/reactor/core/publisher/FluxConcatMap.java	90.42% <0%> (+0.28%)	7% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxReplay.java	84.47% <0%> (+0.31%)	25% <0%> (ø)	⬇️
...eactor/core/publisher/ParallelMergeSequential.java	80.31% <0%> (+0.51%)	7% <0%> (ø)	⬇️
...ava/reactor/core/publisher/EventLoopProcessor.java	80.82% <0%> (+0.51%)	53% <0%> (+1%)	⬆️
...in/java/reactor/core/publisher/FluxWindowWhen.java	81.73% <0%> (+1.44%)	2% <0%> (ø)	⬇️
... and 2 more

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[bsideup]

@simonbasle don't we need to do the same in MonoUsingWhen?

[simonbasle]

@simonbasle don't we need to do the same in MonoUsingWhen?

nope, the MonoUsingWhen operator reuses the same Subscriber.