Apply str() to subject since Header does not have a nice repr
Add nicer logging initialization to tkmail.monitor
Check for X-Amavis-Alert on non-sane messages

A spam email was observed that contained an invalid Message-ID header.
The Python email module reencoded that header differently, causing the
message to fail the sanity check.
The same message happened to trigger an X-Amavis-Alert for an invalid
octet, so we disable the dumping of the full message when emails contain
this header.
Add ResilientSMTPChannel

A spam email was observed that contained an invalid octet, so the
Python smtpd module caused an uncaught UnicodeDecodeError to be raised
when trying to decode the input as UTF-8.

Catch this and log it instead of dropping it on the floor.

As an added benefit, log SMTPChannel messages using the logging module
instead of writing them to the tty (which is the default).
Listen only on localhost