Implemented dynamic domain support

[endreszabo]

What is this

Implemented dynamic domain support that also doubles as a health check endpoint for loadbalancers.
Details:

Dynamic domain support

Traditionally conn.Domain contains the domain that should be the server identifying itself to the connecting clients with during the initial greeting. My PR enabled this field to be dynamic: a function can be specified to generate the value for this field instead of passing the conn.Domain value. It is backward compatible as the greeter will send the value of the conn.Domain field if the dynamic getter function is not set. This feature is useful if a given server handles traffic for more than one endpoint.

Health check support

Greeter function will reject the connection with 554 if the dynamic function returns an error. This is very intentional and is particularly useful with load-balanced setups as in this case the server has an option to tell the health-check-monitoring loadbalancer if they are incapable of receiving traffic. The Conn.handle was also modified to be in conform with RFC5321 regarding proper handling of rejected sessions.

Checkmarks

• implementation
• tests
• documentation: not found anything besides godoc.

[emersion]

This would better be implemented at the backend level.

[endreszabo]

if so, how? Initial greeting banner with the domain is presented by Conn.greet() called by the server that has just Accept()ed the connection and the backend is not even initialized before client issues an EHLO/HELO/LHLO in Conn.handleGreet(). I miss the point where the backend could have any control of this in its lifetime. Please advise.