Add WebsocketOutbound#sendClose() API

[violetagg]

Prepare to send a close frame on subscribe then close the
underlying channel. Any previous publisher will be canceled.

Clarify that nesting NettyOutbound#send* methods is not supported.

Related to #444

[codecov-io]

Codecov Report

Merging #469 into 0.7.x will decrease coverage by 0.2%.
The diff coverage is 30.23%.

@@             Coverage Diff              @@
##              0.7.x     #469      +/-   ##
============================================
- Coverage     68.55%   68.34%   -0.21%     
+ Complexity     1077     1075       -2     
============================================
  Files            73       73              
  Lines          4582     4577       -5     
  Branches        660      661       +1     
============================================
- Hits           3141     3128      -13     
- Misses         1057     1065       +8     
  Partials        384      384

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/reactor/ipc/netty/NettyOutbound.java	80% <ø> (ø)	29 <0> (ø)	⬇️
...or/ipc/netty/http/websocket/WebsocketOutbound.java	100% <ø> (ø)	3 <0> (ø)	⬇️
...or/ipc/netty/http/client/HttpClientOperations.java	56.96% <0%> (+0.11%)	82 <0> (+1)	⬆️
.../ipc/netty/http/client/HttpClientWSOperations.java	65.16% <26.31%> (-11.46%)	19 <0> (ø)
.../ipc/netty/http/server/HttpServerWSOperations.java	60% <50%> (-3.16%)	17 <4> (+3)
...va/reactor/ipc/netty/channel/AbortedException.java	37.5% <0%> (-18.75%)	5% <0%> (-1%)
.../ipc/netty/channel/PooledClientContextHandler.java	66.38% <0%> (-5.05%)	26% <0%> (-2%)
...java/reactor/ipc/netty/channel/ContextHandler.java	74.75% <0%> (-1.95%)	27% <0%> (-1%)
...or/ipc/netty/channel/ChannelOperationsHandler.java	66.01% <0%> (-0.66%)	61% <0%> (-1%)
...a/reactor/ipc/netty/channel/ChannelOperations.java	82.67% <0%> (+0.78%)	48% <0%> (+1%)	⬆️
... and 3 more

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