2.x: Optimize ObservableConcatMapCompletable

[akarnokd]

This PR reduces the overhead in the Observable.concatMapCompletable operator by:

• adding front-queue fusion to eliminate queue allocations (although this didn't improve the performance beyond a +/- 3% noise),
• moving the error and errorMode fields into local variables, saving on the mandatory volatile re-read of other fields,
• adding a scalar shortcut in case the upstream is known to be 0-1.

Benchmark comparison:

i7 4770K, Windows 7 x64, Java 8u162

The PR improves the dedicated case considerably:

and it is now better across the board compared to a toFlowable conversion and the main concatMap:

[codecov]

Codecov Report

Merging #5915 into 2.x will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x   #5915      +/-   ##
===========================================
- Coverage     98.24%   98.2%   -0.04%     
- Complexity     5997    6004       +7     
===========================================
  Files           655     655              
  Lines         43927   43963      +36     
  Branches       6084    6090       +6     
===========================================
+ Hits          43156   43174      +18     
+ Misses          237     233       -4     
- Partials        534     556      +22

Impacted Files	Coverage Δ	Complexity Δ
...perators/mixed/ObservableConcatMapCompletable.java	99.32% <100%> (-0.68%)	7 <6> (+5)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-7.47%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-4.58%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...perators/mixed/ObservableSwitchMapCompletable.java	97.87% <0%> (-2.13%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.02% <0%> (-2.13%)	11% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.78% <0%> (-1.94%)	2% <0%> (ø)
...vex/internal/operators/single/SingleTakeUntil.java	98.27% <0%> (-1.73%)	2% <0%> (ø)
... and 27 more

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