Add the agent HTTP proxy protocol - #1725

A worker does not accept inbound connections, so it dials a pool of
websockets to the edge and the edge multiplexes requests over them. A
stream carries opaque HTTP bytes both ways, so request streaming, SSE
and websocket upgrades work without either side parsing HTTP.
The worker names itself and announces its endpoints in the first frame
of every wire. Registration reaches whichever node the load balancer
picked, which is not the node the wires land on, so it cannot carry
this.

The cloud side of this is the controller: the node a worker dials and
that proxies requests to it. Nothing else in the tree calls it an edge.