2.x: Improve Completable.delay operator internals

[akarnokd]

This PR improves the Completable.delay() operator internals, reducing allocation and indirection.

[codecov]

Codecov Report

Merging #6096 into 2.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6096      +/-   ##
============================================
+ Coverage     98.26%   98.26%   +<.01%     
- Complexity     6194     6198       +4     
============================================
  Files           667      667              
  Lines         44858    44862       +4     
  Branches       6211     6213       +2     
============================================
+ Hits          44081    44085       +4     
- Misses          234      235       +1     
+ Partials        543      542       -1

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/completable/CompletableDelay.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...o/reactivex/internal/schedulers/SchedulerWhen.java	97.64% <0%> (-2.36%)	5% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.44% <0%> (-1.71%)	11% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.2% <0%> (-1.45%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	96.93% <0%> (-1.03%)	2% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	86.58% <0%> (-0.64%)	3% <0%> (ø)
... and 22 more

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[davidmoten]

LGTM

[artem-zinnatullin]

neat