http: set socket timeout when socket connects

`request.setTimeout()` calls `socket.setTimeout()` as soon as a socket
is assigned to the request. This makes the `timeout` event to be
emitted on the request even if the underlying socket never connects.

This commit makes `socket.setTimeout()` to be called only when the
underlying socket is connected.

PR-URL: https://github.com/nodejs/node/pull/8895


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>