net/http: Transport socket late binding

Implement what Chrome calls socket "late binding". See:
https://insouciant.org/tech/connection-management-in-chromium/

In a nutshell, if our HTTP client needs a TCP connection to a
remote host and there's not an idle one available, rather than
kick off a dial and wait for that specific dial, we instead
kick off a dial and wait for either our own dial to finish, or
any other TCP connection to that same host to become
available.

The implementation looks like a classic "Learning Go
Concurrency" slide.

Chrome's commit and numbers:
http://src.chromium.org/viewvc/chrome?view=rev&revision=36230

R=golang-dev, daniel.morsing, adg
CC=golang-dev
https://codereview.appspot.com/7587043