This commit adds a new method for TLS sockets that returns the
negotiated protocol version.
PR-URL: https://github.com/nodejs/node/pull/4995
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
