Refactoring of SMTP backend, using Net::SSLGlue::SMTP.

I refactored the SMTP backend to have less code duplication. Also, I
moved to Net::SSLGlue::SMTP for the TLS / SSL connections instead of
Net::SMTP::SSL and the very ugly named Net::SMTP::TLS::ButMaintained.

The additional benefit this brings is that it allows us to NOT
verify the SSL certificates, see the related bug report here:
http://bugs.otrs.org/show_bug.cgi?id=8982

And of course this change would also allow us to DO check the
certificates, although this requires some more engineering work
if we want to be able to store exceptions and such.