[michalovits]

Keep a reference to the channel in ChannelMap::processReceived. This is just defence. It makes it impossible to invalidate the channel object during this call.

Currently that can't happen anyway:

• A connection owns the channels through ChannelMap and Connection::readHandler keeps the connection alive.
• ChannelMap::removeChannel only runs after Channel::processReceived returns (if it returns CLEANUP).
• Connection::socketShutdown is only called from socket and timer callbacks, frames on channel 0 and scheduled closes, none of which can run under Channel::processReceived.
• The only user code that runs inline is a continuation on createProducerAsync or createConsumerAsync. If it destroys the producer, consumer or connection objects, the close is scheduled to the event loop for later.