http: use 'connect' event only if socket is connecting

Fixes a bug that prevented `ClientRequest.prototype.setTimeout()` from
working properly when the socket was reused for multiple requests.

Fixes: https://github.com/nodejs/node/issues/16716
Refs: https://github.com/nodejs/node/pull/8895
PR-URL: https://github.com/nodejs/node/pull/16725


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>