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

Issue #223 Force closing underlying physical connection whenever an SSE response is received on it.
Issue #225 Now the connection close is triggered after the content completes or the sole subscriber of the content unsubscribes.
Issue #226 Flushing response when `RequestHandler` emits an error or throws an unexpected error.
Issue #214 `HttpServerRequest` should provide access to the underlying `Channel`.
Issue #228 Provided `getChannel()` in `DefaultChannelWriter` instead of `getChannelHandlerContext()`.
Issue #230 FlatResponseOperator does not emit any item if there is no content.