refactor protocol

[kevinkreiser]

it seems the simple nature of the protocol defintions is great for netstrings but completely unsufficient for http (which is a very interesting use case). we should refactor that to be something more like:

request - an object that the server pushes down into the proxy/worker layers
response - an object that a worker pushes back up to the server when finished with its work
stream_request - used by the server to collect bytes from a client and can break itself into multiple request objects
stream_response - used by the client to collect bytes from a server and can break itself into multiple response objects