2.x: Final planned Observable marble additions/fixes

[akarnokd]

This is the final part of the Observable marble fixes and additions as planned in #5319.

• toList(Callable)
• toFlowable(BUFFER)
• toFlowable(DROP)
• toFlowable(LATEST)
• toFlowable(ERROR)
• toFlowable(MISSING)
• unsubscribeOn
• Fixed position and spacing of the marbles in the zipWith overloads (no image change).

Images in #5319 (comment) .

[codecov]

Codecov Report

Merging #5805 into 2.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5805      +/-   ##
============================================
+ Coverage     96.29%   96.34%   +0.05%     
+ Complexity     5817     5814       -3     
============================================
  Files           634      634              
  Lines         41647    41647              
  Branches       5776     5776              
============================================
+ Hits          40103    40124      +21     
+ Misses          612      591      -21     
  Partials        932      932

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	509 <0> (ø)	⬇️
...tivex/internal/schedulers/InstantPeriodicTask.java	61.11% <0%> (-5.56%)	7% <0%> (-2%)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.47% <0%> (-2.72%)	2% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...java/io/reactivex/processors/UnicastProcessor.java	94.64% <0%> (-1.2%)	61% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.71% <0%> (-0.97%)	2% <0%> (ø)
... and 29 more

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