https: use `servername` in agent key

https requests with different SNI values should not be sent over the
same connection, even if the `host` is the same. Server may want to
present different certificate or route the incoming TLS connection
differently, depending on the received servername extension.

Fix: https://github.com/nodejs/node/issues/3940
PR-URL: https://github.com/nodejs/node/pull/4389


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>