Disable ipv6, ident lookups, and smtp sync enforce

Disable ipv6, ident lookups, and smtp sync enforce

ident checks are pointless and just caused the banner to appear more
slowly -- now we can perform an entire mail transaction in under two
seconds vs 7-8s just for the banner to appear on deleuze.

exim was trying to contact ipv6 hosts, disabled ipv6 until we have
ipv6.

Disabling smtp sync enforcement -- hopefully will not help any
spammers connect, and it allows thunderbird to probe our server
without failing resulting in a better member experience.