3.x: elementAt, first - constrain upstream requests

[davidmoten]

As discussed in #6569 this PR constrains upstream requests for the elementAt and first overloads. Other operators will be covered in other PRs.

[codecov]

Codecov Report

Merging #6620 into 3.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6620      +/-   ##
============================================
+ Coverage     98.13%   98.17%   +0.03%     
- Complexity     6187     6188       +1     
============================================
  Files           678      678              
  Lines         44792    44792              
  Branches       6173     6173              
============================================
+ Hits          43956    43973      +17     
+ Misses          290      281       -9     
+ Partials        546      538       -8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	565 <0> (ø)	⬇️
...nal/operators/flowable/FlowableElementAtMaybe.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...al/operators/flowable/FlowableElementAtSingle.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.92%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.25% <0%> (-1.15%)	2% <0%> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-0.54%)	5% <0%> (ø)
... and 15 more

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[akarnokd]

Could you hold up your subsequent PRs until the refactor of the packages?

[davidmoten]

Could you hold up your subsequent PRs until the refactor of the packages?

Sure.