Use nrepl hooks in cider

cider-jack-in or cider trigger autoload of cider.el, and thus require all of the cider files downto nrepl-client. Instead of doing an autoload on the connection triggers for cider in nrepl-client, we should leverage the fact that cider.el is the likely trigger for loading cider.

I'm less certain about calling the disconnection hook in nrepl-close versus automatically on each sentinel that quits.