Current versions of tor accept USERADDR with or without a port number,
but future versions may become more strict and require the port number.
https://bugs.torproject.org/23080
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
This removes the --tls-cert and --tls-keys options and replaces them
with --acme-hostname and (optional) --acme-email. It uses
https://godoc.org/golang.org/x/crypto/acme/autocert, which is kind of a
successor to https://godoc.org/rsc.io/letsencrypt.
The autocert package only works when the listener runs on port 443. For
that reason, if TOR_PT_SERVER_BINDADDR asks for a port other than 443,
the program will open an *additional* listening port on 443. If there is
an error opening the listener, it is reported through an SMETHOD-ERROR
for the requested address.
The inspiration for this code came from George Tankersley's patch for
meek-server:
https://bugs.torproject.org/18655#comment:8https://github.com/gtank/meek/tree/letsencrypt
