We only seem to use this for knowing whether to try SASL auth.  I suggest we keep this method around (we may want to support LOGIN-DELAY eventually; like if we start getting blacklisted by servers) but stop using it for now.  Pull the STLS logic out and just run it by default.

Have the login method always try the SASL thing just in case (same number of round-trips as doing CAPA always).  Maybe alter the class to keep a list of auth methods to try like ['apop', 'sasl', 'user-pass'].  And we could optimize things by having the prober return the auth method that worked, then our future Pop3Client instantiations we can just .unshift(originalAuthMethodThatWorkedForUs).

What I propose may be slightly overkill, just thinking that since part of the flow for POP3 is constantly dropping the connection, it may be slightly more important to optimize this.  (Also, IMAP servers do throw the capability info at us, so we don't always have to ask for it ourselves...)