http: More useful setTimeout API on server

This adds the following to HTTP:

* server.setTimeout(msecs, callback)
  Sets all new connections to time out after the specified time, at
  which point it emits 'timeout' on the server, passing the socket as an
  argument.
  In this way, timeouts can be handled in one place consistently.
* req.setTimeout(), res.setTimeout()
  Essentially an alias to req/res.socket.setTimeout(), but without
  having to delve into a "buried" object.  Adds a listener on the
  req/res object, but not on the socket.
* server.timeout
  Number of milliseconds before incoming connections time out.
  (Default=1000*60*2, as before.)

Furthermore, if the user sets up their own timeout listener on either the server, the request, or the response, then the default behavior (destroying the socket) is suppressed.

Fix #3460

Review, please: @davepacheco @bnoordhuis @mcavage