Add wrap-with-certificate-cn middleware

This middleware adds a :ssl-client-cn key to the request map if a
:ssl-client-cert is present. This makes for easier certificate
whitelisting (using the cert whitelisting function from pl/kitchensink).
:+1: on adding something to the jvm-ca for this, maybe a new `get-cn-from-x509-certificate`?  @nathanielksmith, you good with doing the work for that - or at least filing a ticket to have that done?