mid_registrar: Restore "default_expires" and "max_expires"
These registrar modparams were left unmaintained / ignored during development.
However, they are still valid and completely separate from
"outgoing_expires". The former two params control ingress expirations,
while the latter one controls egress expirations.
To match the newly introduced defaults, we also increase the default
"outgoing_expires" from 600s to 3600s.