Font-lock deprecated vars

[Malabarba]

This should be pretty simple to do using the same system that font-locks instrumented vars.
Just add :deprecated to the keywords tracked by cider-nrepl's track-state middleware, and add a font-lock entry for it in cider-mode.el similar to the one we use for instrumented vars (the actual face should be different, of course).

[bbatsov]

Yeah, that's a good idea.