Permits all sorts of custom HTTP timeout policies without
adding a new Transport timeout Duration for each combination
of HTTP phases.

This keeps track internally of which TCP connection a given
Request is on, and lets callers forcefully close the TCP
connection for a given request, without actually getting
the net.Conn directly.

Additionally, a future CL will implement res.Body.Close (Issue
3672) in terms of this.

Update #3362
Update #3672

R=golang-dev, rsc, adg
CC=golang-dev
https://golang.org/cl/7372054