2.x: Call the doOn{Dispose|Cancel} handler at most once

[akarnokd]

This PR makes sure the doOnDispose, doOnCancel and doOnLifecycle execute their cancellation handler once.

Resolves: #6268.

[codecov]

Codecov Report

Merging #6269 into 2.x will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #6269      +/-   ##
===========================================
- Coverage     98.24%   98.2%   -0.04%     
+ Complexity     6210    6207       -3     
===========================================
  Files           667     667              
  Lines         44897   44905       +8     
  Branches       6226    6228       +2     
===========================================
- Hits          44108   44098      -10     
- Misses          252     253       +1     
- Partials        537     554      +17

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	541 <0> (ø)	⬇️
...x/internal/observers/DisposableLambdaObserver.java	100% <100%> (ø)	11 <2> (+1)	⬆️
...rnal/operators/flowable/FlowableDoOnLifecycle.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-9.16%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...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%)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.24%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.02% <0%> (-1.71%)	11% <0%> (ø)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
... and 18 more

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