Add ServerDomain member into the ConnectionState struct

[kayrus]

Add server's hostname (domain) into the ConnectionState struct. It can be used for logging purposes.

[emersion]

Hm, can't you just get it from the Server directly?

[kayrus]

when you pass only the state to some internal func, there could be no server struct, only the state. introducing a new struct wrapper around that contains ServerDomain member looks like an overkill... :\

[emersion]

ConnectionState has been dropped, superseded by NewSession.