Split HttpTransport in two.

The new (awkwardly-named) HttpConnection class now owns the
socket, and stays around between HTTP requests. It matches
SpdyConnection in lifecycle: it is tied to the socket.

The HTTP transport class is a dumb adapter that adapts the
protocol-specific decisions (chunked-encoding, content-length
stuff).

This should make it possible to make significant simplifications
to how connection recycling works: in a follow up change the
HttpConnection class will know when it can be recycled and take
that action automatically.