2.x: Add Flowable.concatMapCompletable{DelayError} operator

[akarnokd]

This PR adds the Flowable.concatMapCompletable and Flowable.concatMapCompletableDelayError operators as requested by #4853.

The marbles will be updated in a separate PR.

[codecov]

Codecov Report

Merging #5871 into 2.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               2.x    #5871      +/-   ##
===========================================
- Coverage     96.5%   96.47%   -0.03%     
- Complexity    5852     5864      +12     
===========================================
  Files          646      647       +1     
  Lines        42640    42766     +126     
  Branches      5910     5933      +23     
===========================================
+ Hits         41148    41258     +110     
- Misses         579      593      +14     
- Partials       913      915       +2

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	539 <6> (+6)	⬆️
.../operators/mixed/FlowableConcatMapCompletable.java	100% <100%> (ø)	2 <2> (?)
...ex/internal/operators/flowable/FlowableCreate.java	90% <0%> (-7.42%)	6% <0%> (ø)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	88.23% <0%> (-3.27%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	94.27% <0%> (-1.77%)	11% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-1.71%)	2% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	98.3% <0%> (-1.7%)	11% <0%> (ø)
... and 25 more

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