2.x: inline disposability in Obs.concatMap(Completable)

[akarnokd]

The inner Observer type and the SequentialDisposable of the concatMap and concatMapCompletable can be combined into a single instance, saving on indirection and allocation.

(I've noticed this when reviewing #5649.)

[codecov]

Codecov Report

Merging #5652 into 2.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5652      +/-   ##
============================================
- Coverage     96.24%   96.21%   -0.03%     
+ Complexity     5851     5848       -3     
============================================
  Files           635      635              
  Lines         41652    41651       -1     
  Branches       5768     5768              
============================================
- Hits          40087    40074      -13     
- Misses          622      626       +4     
- Partials        943      951       +8

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/observable/ObservableConcatMap.java	97.52% <100%> (ø)	5 <0> (ø)	⬇️
...ors/observable/ObservableConcatMapCompletable.java	96.29% <100%> (-0.04%)	2 <0> (ø)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <0%> (-3.28%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <0%> (-1.84%)	39% <0%> (-1%)
...java/io/reactivex/processors/PublishProcessor.java	98.29% <0%> (-1.71%)	45% <0%> (-1%)
... and 20 more

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