3.x: [Java 8] Add ParallelFlowable operators + cleanup

[akarnokd]

Add Java 8 operators to the ParallelFlowable class:

• mapOptional + 2
• flatMapStream + 1
• collect

Also for symmetry, the flatMapIterable operator has also been added.

Cleanups:

• Add missing @since tags, unify tag location.
• Use diamond in parallel classes.
• Fix error messages in the parallel-try operators.
• Improve validation messages with at entries so the IDE can navigate to the problematic line.
• Add some unit tests to improve coverage of some existing parallel operators.

[codecov]

Codecov Report

Merging #6798 into 3.x will decrease coverage by 0.01%.
The diff coverage is 99.01%.

@@ Coverage Diff @@ ## 3.x #6798 +/- ## ============================================ - Coverage 98.23% 98.22% -0.02%  - Complexity 6340 6374 +34  ============================================ Files 706 711 +5 Lines 45846 46217 +371 Branches 6280 6329 +49 ============================================ + Hits 45039 45398 +359  - Misses 283 297 +14  + Partials 524 522 -2

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	177 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	553 <0> (ø)	⬇️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	577 <0> (ø)	⬇️
...ava3/internal/operators/parallel/ParallelPeek.java	98.94% <100%> (ø)	5 <0> (ø)	⬇️
...vex/rxjava3/internal/jdk8/ParallelMapOptional.java	100% <100%> (ø)	6 <6> (?)
...va3/internal/operators/parallel/ParallelRunOn.java	97.1% <100%> (ø)	8 <0> (ø)	⬇️
...internal/operators/parallel/ParallelFilterTry.java	97.34% <100%> (ø)	6 <0> (ø)	⬇️
...a3/internal/operators/parallel/ParallelFilter.java	97.59% <100%> (ø)	6 <0> (ø)	⬇️
...al/operators/flowable/FlowableFlattenIterable.java	100% <100%> (ø)	5 <1> (+1)	⬆️
...nternal/operators/parallel/ParallelSortedJoin.java	95.62% <100%> (+0.72%)	2 <1> (ø)	⬇️
... and 48 more

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