2.x: Fix concatMapSingle & concatMapMaybe dispose-cleanup crash

[akarnokd]

This PR fixes the accidental logical mistake in the cancel/dispose logic of the new concatMapSingle and concatMapMaybe operators of both Flowable and Observable where the internal queue cleanup should happen in a serialized fashion only, which is the state when the work-in-progress counter changes from 0 to 1 ensured by a == check.

Fixes #5927

[akarnokd]

This is severe enough to release 2.1.12 as a hotfix version ASAP, although the mistakes are in the newly introduced operators. I've done a quick search for this pattern and doesn't happen in older code luckily. Also this is on the dispose path and doesn't really the performance in the normal operation mode (i.e., when data is allowed to flow through till the end without cancellation).

[codecov]

Codecov Report

Merging #5928 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5928      +/-   ##
============================================
+ Coverage     98.22%   98.25%   +0.02%     
- Complexity     6017     6019       +2     
============================================
  Files           656      656              
  Lines         44040    44040              
  Branches       6102     6102              
============================================
+ Hits          43260    43270      +10     
+ Misses          237      231       -6     
+ Partials        543      539       -4

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/mixed/FlowableConcatMapMaybe.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...rnal/operators/mixed/ObservableConcatMapMaybe.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...nal/operators/mixed/ObservableConcatMapSingle.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...ernal/operators/mixed/FlowableConcatMapSingle.java	100% <100%> (ø)	2 <0> (ø)	⬇️
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	93.04% <0%> (-2.14%)	5% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
... and 26 more

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