http: destroy sockets after keepAliveTimeout

Implement server.keepAliveTimeout in addition to server.timeout to
prevent temporary socket/memory leaking in keep-alive mode.

PR-URL: https://github.com/nodejs/node/pull/2534


Author: Timur Shemsedinov <timur.shemsedinov@gmail.com>
Author: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>