2.x: more eager cancellation in flatMapX

[akarnokd]

The FlatMapX tests kept failing because there was a race between the cancellation and the disposing of their inner set: the cancel may have interrupted the test wait and ended up in the inner subscribe before set.isDisposed() would become true in the dispose() call of the operator that run on the main thread. The change adds/uses a boolean field that gets set first and is checked before the inner are subscribed.

[codecov]

Codecov Report

Merging #5422 into 2.x will decrease coverage by 0.14%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##                2.x    #5422      +/-   ##
============================================
- Coverage     96.12%   95.98%   -0.15%     
+ Complexity     5785     5781       -4     
============================================
  Files           630      630              
  Lines         41232    41236       +4     
  Branches       5728     5728              
============================================
- Hits          39635    39581      -54     
- Misses          627      650      +23     
- Partials        970     1005      +35

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.71% <0%> (-1.45%)	2 <0> (ø)
.../operators/observable/ObservableFlatMapSingle.java	92.53% <0%> (-3.74%)	2 <0> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	94.11% <0%> (-2.62%)	2 <0> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.65% <0%> (ø)	2 <0> (ø)	⬇️
...vable/ObservableFlatMapCompletableCompletable.java	95.83% <50%> (-1.35%)	3 <0> (ø)
...lowable/FlowableFlatMapCompletableCompletable.java	96.38% <50%> (-1.18%)	3 <0> (ø)
...ators/observable/ObservableFlatMapCompletable.java	95.94% <50%> (-1.32%)	2 <0> (ø)
...operators/flowable/FlowableFlatMapCompletable.java	94.11% <50%> (-1.13%)	2 <0> (ø)
...al/operators/observable/ObservableSampleTimed.java	88.33% <0%> (-10%)	3% <0%> (ø)
...ternal/operators/flowable/FlowableSubscribeOn.java	91.52% <0%> (-6.78%)	2% <0%> (ø)
... and 39 more

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