1.x: add unsubscribeOn to Single type (#5138)

[yshrsmz]

resolve #5138

add unsubscribeOn to Single type.

[codecov]

Codecov Report

Merging #5146 into 1.x will increase coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                1.x    #5146      +/-   ##
============================================
+ Coverage     84.29%   84.37%   +0.07%     
- Complexity     2875     2878       +3     
============================================
  Files           290      290              
  Lines         18091    18107      +16     
  Branches       2475     2475              
============================================
+ Hits          15250    15277      +27     
+ Misses         1973     1960      -13     
- Partials        868      870       +2

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/rx/Single.java	73.86% <100%> (+3.29%)	82 <1> (+1)	✅
...in/java/rx/internal/schedulers/SleepingAction.java	55.55% <0%> (-5.56%)	2% <0%> (-1%)
.../java/rx/internal/operators/BackpressureUtils.java	68.18% <0%> (-4.55%)	27% <0%> (-2%)
...ain/java/rx/internal/schedulers/SchedulerWhen.java	83.78% <0%> (-4.06%)	4% <0%> (ø)
...ava/rx/internal/operators/OperatorMaterialize.java	85.24% <0%> (-3.28%)	3% <0%> (ø)
...ava/rx/internal/util/BackpressureDrainManager.java	66.66% <0%> (-1.24%)	20% <0%> (-1%)
...in/java/rx/internal/operators/OperatorPublish.java	78.9% <0%> (+0.42%)	8% <0%> (ø)	❌
.../rx/internal/schedulers/CachedThreadScheduler.java	89.32% <0%> (+0.97%)	6% <0%> (ø)	❌
...n/java/rx/subscriptions/CompositeSubscription.java	77.92% <0%> (+1.29%)	25% <0%> (+1%)	✅
...n/java/rx/subjects/SubjectSubscriptionManager.java	82.14% <0%> (+1.42%)	16% <0%> (+1%)	✅
... and 3 more

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