3.x: Promote Beta/Experimental to Standard

[akarnokd]

Promote the following experimental operators to standard:

• Completable.delaySubscription(long, TimeUnit [, Scheduler])
• Completable.materialize()
• Flowable.dematerialize(Function)
• Maybe.doOnTerminate()
• Maybe.materialize()
• Observable.dematerialize(Function)
• Single.dematerialize(Function)
• Single.materialize()

There were no Beta operators in 2.2.x.

[codecov]

Codecov Report

Merging #6537 into 3.x will increase coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               3.x    #6537      +/-   ##
===========================================
+ Coverage     98.2%   98.28%   +0.08%     
  Complexity    6199     6199              
===========================================
  Files          680      680              
  Lines        44976    44976              
  Branches      6202     6202              
===========================================
+ Hits         44168    44204      +36     
+ Misses         259      241      -18     
+ Partials       549      531      -18

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/mixed/MaterializeSingleObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	572 <0> (ø)	⬇️
.../operators/completable/CompletableMaterialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...vex/internal/operators/maybe/MaybeMaterialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	11 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	118 <0> (ø)	⬇️
...x/internal/operators/single/SingleMaterialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	547 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	148 <0> (ø)	⬇️
...internal/operators/single/SingleDematerialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
... and 30 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 62d2b31...2cf938f. Read the comment docs.