Properly encode IDN domain names for SRV lookups.

Before Twisted 12.3.0, the SRV lookup done for outgoing
connections succeeded if passed a unicode string with all-ASCII
code points. A recent change made the DNS code more strict, and
only byte string are accepted as domain name.  See
http://twistedmatrix.com/trac/ticket/6245 for details.

This change makes sure domain names are encoded to their ASCII
Compatible Equivalent (ACE) version before passing the resulting
byte string to `twisted.names.srvconnect.SRVConnector`, as per
RFC 3490.

Note that while connecting to servers with an IDN domain name
now works properly, authentication using MD5-Digest SASL mechanism
will fail until http://twistedmatrix.com/trac/ticket/5066 has been
resolved.

Fixes: #77.