2.x: Cleanup, coverage and related component fixes

[akarnokd]

This PR improves the coverage of various components and contains the following additional fixes:

• Remove a very unlikely cancellation check in FlowableConcatMapEager.
• Improve backpressure tracking in FlowablePublishMulticast to avoid atomic decrement per items emitted (part of publish(Function)).
• Improve check for empty or terminated state in FlowablePublishMulticast by using the array length.
• UnicastProcessor termination to use a one-step getAndSet
• BehaviorSubject termination to use a one-step getAndSet
• Remove impossible try-finally in BehaviorSubject.setCurrent as the body could never crash.

[codecov]

Codecov Report

Merging #5889 into 2.x will increase coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5889      +/-   ##
============================================
+ Coverage     97.49%   97.57%   +0.07%     
- Complexity     5946     5956      +10     
============================================
  Files           655      655              
  Lines         43900    43897       -3     
  Branches       6108     6106       -2     
============================================
+ Hits          42799    42831      +32     
+ Misses          339      333       -6     
+ Partials        762      733      -29

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/flowable/FlowableConcatMapEager.java	98.24% <ø> (-0.03%)	2 <0> (ø)
...l/operators/flowable/FlowablePublishMulticast.java	100% <100%> (+6.52%)	2 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	96.23% <100%> (-2.18%)	54 <1> (ø)
...java/io/reactivex/processors/UnicastProcessor.java	100% <100%> (+4.16%)	68 <2> (+6)	⬆️
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-3.14%)	59% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-1.45%)	2% <0%> (ø)
... and 31 more

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