2.x: Improve coverage and fix small mistakes/untaken paths in operators

[akarnokd]

This PR improves the coverage of RxJava while adjusting some code paths and fixing other types of smaller bugs.

• Fix Maybe.merge(Publisher) to define a 1 element buffer only.
• Fix Maybe.mergeDelayError(Publisher) to use the dedicated FlowableFlatMapPublisher similar to the plain merge().
• In Flowable.flatMap, checking for empty or cancelled arrays in removeInner() can be replaced with a length check.
• Make sure in Flowable.reduce(seed, f) the terminal events can't be called a second time if the reducer crashes.
• Turn the FlowableReplay.MultiCastPublisher into a Flowable and rename it to MulticastFlowable.
• Fix FlowableWindowBoundary not cancelling the upstream on a missing backpressure case, causing NullPointerException.
• Remove unused override of accept() in FlowableWindowBoundary.
• Remove the ineffective done flag from OperatorWindowBoundaryOpenSubscriber.
• Replace the timer CASs with the replace() call in FlowableWindowTimed.
• Remove the unused RepeatWhen, ErrorMapperFilter and RetryWhen components from ObservableInternalHelper.
• Make sure the value is cleared at most once in ObservableReduceSeedSingle.
• Simplify ObservableWindowBoundarySelector's inner consumer's onNext handling.
• Simplify FlowableWindowBoundarySelector's inner consumer's onNext handling.
• Inline the finally actions in InstantPeriodicTask.call().
• Fix InstantPeriodicTask.setFirst and setRest to return from the loop when the task has been cancelled to prevent excess looping and overwriting the CANCELLED indicator.
• Inline the finally actions in ScheduledDirectPeriodicTask.call().
• Slight adjustment of crash propagation in the SchedulerWhen constructor.
• In BehaviorProcessor, checking for empty or cancelled arrays in remove() can be replaced with a length check.
• In BehaviorSubject, checking for empty or cancelled arrays in remove() can be replaced with a length check.

[codecov]

Codecov Report

Merging #5883 into 2.x will increase coverage by 0.78%.
The diff coverage is 98.07%.

@@             Coverage Diff              @@
##                2.x    #5883      +/-   ##
============================================
+ Coverage     96.61%   97.39%   +0.78%     
- Complexity     5921     5978      +57     
============================================
  Files           657      657              
  Lines         43986    43960      -26     
  Branches       6132     6128       -4     
============================================
+ Hits          42497    42817     +320     
+ Misses          582      352     -230     
+ Partials        907      791     -116

Impacted Files	Coverage Δ	Complexity Δ
...ators/flowable/FlowableWindowBoundarySupplier.java	91.5% <ø> (+12.41%)	2 <0> (ø)	⬇️
...s/observable/ObservableWindowBoundarySupplier.java	89.62% <ø> (+8.14%)	2 <0> (ø)	⬇️
...operators/observable/ObservableInternalHelper.java	100% <ø> (+13.04%)	15 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <100%> (+5.82%)	60 <0> (ø)	⬇️
...s/observable/ObservableWindowBoundarySelector.java	90.79% <100%> (+5.85%)	2 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	98.41% <100%> (+12.16%)	54 <0> (ø)	⬇️
...ators/flowable/FlowableWindowBoundarySelector.java	86.78% <100%> (+8.28%)	2 <0> (ø)	⬇️
...ex/internal/operators/flowable/FlowableReplay.java	91.75% <100%> (+0.84%)	22 <1> (ø)	⬇️
...erators/observable/ObservableReduceSeedSingle.java	100% <100%> (+15.38%)	2 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <100%> (ø)	171 <2> (ø)	⬇️
... and 69 more

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