Revisit write operations

[smaldini]

Fix #572 - #654

• send(Publisher) : supports fusion, Mono, flush on write burst. Flush API will be revisited in 0.9
• Interleaving send(Publisher) is now possible, avoiding livelock with recursive send/write. Use a single send with a Processor to serialize multiple sends.
• HTTP send(Publisher) if Mono, a content length header will be automatically used and the response will be flushed as once by default, consistent with the client Get and Head requests as of now. If chunked transfer is still expected, use of Flux instead of Mono is necessary.
• default TCP options have been tuned down and smaller sndbuf/rcvbuf will result

[codecov-io]

Codecov Report

Merging #725 into 0.8.x will increase coverage by 0.23%.
The diff coverage is 72.72%.

@@             Coverage Diff              @@
##              0.8.x     #725      +/-   ##
============================================
+ Coverage     65.73%   65.96%   +0.23%     
+ Complexity     1401     1362      -39     
============================================
  Files           130      132       +2     
  Lines          6624     6353     -271     
  Branches        889      834      -55     
============================================
- Hits           4354     4191     -163     
+ Misses         1774     1716      -58     
+ Partials        496      446      -50

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/reactor/netty/NettyPipeline.java	100% <ø> (ø)	1 <0> (ø)	⬇️
.../reactor/netty/http/server/HttpServerResponse.java	100% <ø> (ø)	3 <0> (-1)	⬇️
...eactor/netty/http/websocket/WebsocketOutbound.java	100% <ø> (ø)	2 <0> (-1)	⬇️
...r/netty/http/client/WebsocketClientOperations.java	67.05% <0%> (-0.8%)	19 <0> (ø)
src/main/java/reactor/netty/FutureMono.java	54.28% <0%> (-14.89%)	4 <0> (-4)
...eactor/netty/http/server/HttpServerOperations.java	72.36% <100%> (+2.76%)	62 <2> (+3)	⬆️
...reactor/netty/http/client/HttpClientFinalizer.java	92.3% <100%> (ø)	22 <1> (ø)	⬇️
...r/netty/http/server/WebsocketServerOperations.java	64.63% <100%> (+1.21%)	20 <1> (+2)	⬆️
src/main/java/reactor/netty/udp/UdpServer.java	47.36% <100%> (-2.64%)	22 <0> (ø)
src/main/java/reactor/netty/NettyOutbound.java	81.25% <100%> (-2.09%)	18 <1> (-2)
... and 22 more

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[smaldini]

There will be further improvement in the next iteration, notably around single send, http message creation and more send aliases (encoder, source cleanup)