1.x: fix premature cleanup in AsyncOnSubscribe when the last Observable is still running

[yurgisbaykshtis]

The issue is caused by premature cleanup when onCompleted is emitted from the callback function which may happen while a requested async sequence is still not completed.
The unit test is provided which will fail without the fix.
More cases are provided in the issue #5429.

[akarnokd]

Thanks for looking into this.

[codecov]

Codecov Report

Merging #5430 into 1.x will decrease coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                1.x    #5430      +/-   ##
============================================
- Coverage     84.38%   84.32%   -0.07%     
+ Complexity     2887     2884       -3     
============================================
  Files           291      291              
  Lines         18129    18129              
  Branches       2479     2479              
============================================
- Hits          15298    15287      -11     
- Misses         1965     1972       +7     
- Partials        866      870       +4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/rx/observables/AsyncOnSubscribe.java	68.35% <100%> (+1.68%)	5 <0> (ø)	⬇️
...java/rx/internal/schedulers/ExecutorScheduler.java	77.46% <0%> (-4.23%)	2% <0%> (ø)
...ava/rx/internal/operators/OperatorMaterialize.java	85.24% <0%> (-3.28%)	3% <0%> (ø)
...n/java/rx/subscriptions/CompositeSubscription.java	74.02% <0%> (-2.6%)	24% <0%> (ø)
...ternal/operators/OperatorOnBackpressureLatest.java	80.95% <0%> (-1.91%)	3% <0%> (ø)
...c/main/java/rx/observables/BlockingObservable.java	85.61% <0%> (-1.44%)	37% <0%> (-1%)
...main/java/rx/internal/operators/OperatorMerge.java	86.37% <0%> (-1.16%)	7% <0%> (ø)
...in/java/rx/internal/operators/OperatorPublish.java	78.48% <0%> (-0.85%)	8% <0%> (ø)
src/main/java/rx/Completable.java	83.24% <0%> (-0.13%)	102% <0%> (-1%)
.../rx/internal/schedulers/CachedThreadScheduler.java	89.32% <0%> (+0.97%)	6% <0%> (ø)	⬇️
... and 2 more

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[akarnokd]

Will take this for now. The whole operator needs an overhaul otherwise.