Provide Uni implementation to avoid abusing from the emitter

[cescoffier]

Fix #384

• Provides implementation for Uni with state
• Make Uni implementation extends AbstractUni directly instead of extending the UniOperator
• Avoid rewrapping a UniSerializedSubscriber in another one.

[codecov]

Codecov Report

Merging #421 (4250e57) into master (92cc951) will increase coverage by 0.31%.
The diff coverage is 87.20%.

@@             Coverage Diff              @@
##             master     #421      +/-   ##
============================================
+ Coverage     89.88%   90.19%   +0.31%     
- Complexity     2765     2782      +17     
============================================
  Files           364      369       +5     
  Lines         10564    10597      +33     
  Branches       1301     1300       -1     
============================================
+ Hits           9495     9558      +63     
+ Misses          558      548      -10     
+ Partials        511      491      -20     

Impacted Files	Coverage Δ	Complexity Δ
...tiny/operators/uni/builders/DefaultUniEmitter.java	93.10% <ø> (ø)	12.00 <0.00> (?)
...y/operators/uni/builders/UniCreateWithEmitter.java	100.00% <ø> (ø)	2.00 <0.00> (?)
...ilders/UniCreateFromDeferredSupplierWithState.java	68.18% <68.18%> (ø)	2.00 <2.00> (?)
...uilders/UniCreateFromCompletionStageWithState.java	78.94% <78.94%> (ø)	2.00 <2.00> (?)
...ators/uni/builders/UniCreateFromItemWithState.java	82.35% <82.35%> (ø)	2.00 <2.00> (?)
...rs/uni/builders/UniCreateFromEmitterWithState.java	88.23% <88.23%> (ø)	2.00 <2.00> (?)
...main/java/io/smallrye/mutiny/groups/UniCreate.java	100.00% <100.00%> (ø)	24.00 <2.00> (-12.00)
...lrye/mutiny/operators/UniSerializedSubscriber.java	92.30% <100.00%> (+0.30%)	15.00 <0.00> (+1.00)
...rye/mutiny/operators/uni/builders/StateHolder.java	100.00% <100.00%> (ø)	4.00 <4.00> (?)
...ors/uni/builders/UniCreateFromCompletionStage.java	100.00% <100.00%> (ø)	7.00 <1.00> (?)
... and 25 more

[jponge]

Great!