Today, we pick the domain used to send the DSN from based on what we
presented to the client at EHLO time, which itself may be based on the
TLS negotiation (which is not necessarily trusted).
This is complex, not necessarily correct, and involves passing the
domain around through the queue and persisting it in the items.
So this patch simplifies that handling by always using the main domain
as specified by the configuration.
This patch moves chasquid's Server and Conn structures to their own
smtpsrv package, to make chasquid.go a bit more readable. It also helps
clarify the relation between Server and Conn.
There are no functional changes.
Note that git can still track the history across this commit (e.g. git
gui blame shows the right data).
