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).
We've been trying to push all stuff like this (calling methods defined on BC classes/interfaces) into jvm-ca.  This might be a bit better if instead we added a `cert->cn` function in jvm-ca, and then just used that in the middleware below.