2.x: Fix concatEager to dispose sources & clean up properly.

[akarnokd]

This PR fixes the concatMapEager operator (which drives the static variants as well) to properly dispose and clean up the state of the operator upon cancelling/disposing the sequence.

• Flowable.concatMapEager did not cancel the current active inner consumer, only the ones coming after
• Observable.concatMapEager did not dispose the main source of the inner observables.

Both variants have received the same two unit tests to verify their behavior.

Fixes: #6404

[codecov]

Codecov Report

Merging #6405 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6405      +/-   ##
============================================
- Coverage     98.24%   98.22%   -0.02%     
  Complexity     6294     6294              
============================================
  Files           675      675              
  Lines         45156    45166      +10     
  Branches       6239     6242       +3     
============================================
+ Hits          44365    44366       +1     
- Misses          247      253       +6     
- Partials        544      547       +3

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/flowable/FlowableConcatMapEager.java	98.85% <100%> (+0.02%)	2 <0> (ø)	⬇️
...operators/observable/ObservableConcatMapEager.java	99.47% <100%> (+0.01%)	2 <0> (ø)	⬇️
.../io/reactivex/internal/schedulers/IoScheduler.java	89.36% <0%> (-4.26%)	9% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.19%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	96.2% <0%> (-2.11%)	11% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	96.12% <0%> (-1.62%)	6% <0%> (ø)
... and 26 more

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