Enhances FluxReceiver internals

[OlegDokuka]

This PR fixes the improper behavior of FluxReceiver when subscription sending might be offloaded to a separate thread (which contradicts to the ReactiveStreams Spec, for more info see -> reactive-streams/reactive-streams-jvm#486 (comment))

Also, since all actions of the FluxReceive class (includes drainReceiver, cancelReceiver, cleanQueue, onInboundXXX) happens within an EventLoop, it makes no sense to use volatile WIP which brings extra overhead. The same applies to SpScUnboundedQueue which can be replaced with the same non-thread-safe analog since there is no racing between offering and draining of data

Signed-off-by: Oleh Dokuka [email protected]

[codecov-commenter]

Codecov Report

Merging #1185 into master will decrease coverage by 0.07%.
The diff coverage is 59.37%.

@@             Coverage Diff              @@
##             master    #1185      +/-   ##
============================================
- Coverage     73.41%   73.34%   -0.08%     
+ Complexity     1782     1781       -1     
============================================
  Files           131      131              
  Lines          8185     8186       +1     
  Branches       1125     1128       +3     
============================================
- Hits           6009     6004       -5     
- Misses         1551     1557       +6     
  Partials        625      625              

Impacted Files	Coverage Δ	Complexity Δ
...eactor/netty/http/client/HttpClientOperations.java	66.84% <33.33%> (-0.36%)	84.00 <0.00> (ø)
...c/main/java/reactor/netty/channel/FluxReceive.java	81.10% <59.25%> (-1.85%)	70.00 <27.00> (-1.00)
...tor/netty/http/client/Http2ConnectionProvider.java	79.75% <100.00%> (ø)	10.00 <0.00> (ø)
...eactor/netty/http/server/HttpServerOperations.java	72.83% <100.00%> (-0.11%)	69.00 <1.00> (ø)

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[violetagg]

@simonbasle PTAL

[violetagg]

@OlegDokuka I squashed and applied the change to 0.9.x branch (5e4bc03) and then forward merge to master.
Thanks for the PR.