Add a DirCache for certificates under TOR_PT_STATE_LOCATION.

This way, we don't lose state of certificates every time the process is
restarted. There's a possibility, otherwise, that if you have to restart
the server rapidly, you might run into Let's Encrypt rate limits and be
unable to create a cert for a while.
https://godoc.org/rsc.io/letsencrypt#hdr-Persistent_Storage