Fixes #223 (Client connections receiving SSE responses should never be pooled & reused)

Channel attribute "rxnetty_http_discard_connection" is used by `ObservableConnection` to discard a connection on close. SSE Inbound handler sets this attribute as a connection on which SSE response has started should never be pooled.
Fixes #226 #225

 Issue 226: Flushing response when `RequestHandler` emits an error or throws an unexpected error.
 Issue 225: Connection close for HttpClient.submit() is now done when the content is completed or the (only) subscriber to the content subject unsubscribes.
Invoking onUnsubscribeAction for dispose also.

If no one subscribes to the UnicastContentSubject, for HttpClientResponse, the connection would never be closed.

For this reason we should also call the onUnubscribe action when the content is disposed.
Fixes #214 #228

Issue #214 Provde getChannel() method in HttpServerRequest
Issue #228 Provide getChannel() method in DefaultChannelWriter and deprecate getChannelHandlerContext()
Fixes #230 (FlatResponseOperator does not emit any item if there is no content.)

Also, fixed the test failure for previous PR
Merge pull request #227 from NiteshKant/0.x

Fixes #223 #225 #226 #214 #228 #230