Remove a race condition from the WebSocket upgrade

[pushkarnk]

An upgrade to WebSocket invokes three handlers in an order. First, the
shouldUpgrade handler supplied by the user is run to decide if an upgrade
must go through. This handler also supplies additional headers to be sent in
the response. Next, after the WebSocket upgrader is done upgrading the
pipeline, the completionHandler is called. We remove Kitura-NIO's
HTTPRequestHandler here. Next, the upgradePipelineHandler is invoked. This
handler allows us to add all the WebSocket related ChannelHandlers.

For an undocumented reason, we saved the ChannelHandlerContext received by
the completionHandler in the HTTPServer and later used it upgrade the
pipeline in upgradePipelineHandler. This can easily lead to a race condition
where we saved the ChannelHandlerContext for a connection, into the
HTTPServer but before it could be used in upgradePipelineHandler, it was
overwritten by the upgrade happening on another connection. Consequently, we
never upgraded the pipeline of the former connection. This could lead to
different kinds of failures.

The upgradePipelineHandler has Channel as one of its parameters. Hence
there is no need to store the ChannelHandlerContext for use in this closure.
Consequently, we have to use a Channel to initialize an HTTPServerRequest,
which, in turn, is modified to accept a Channel instead of a
ChannelHandlerContext.

[bridger]

Great find!