director: Change request callback to take mail_host parameter This allows accessing the IP address both as struct and as string without any conversions.

director: Remember backends' hostnames and send them in login reply. This allows login processes to verify the remote server's hostname in SSL certificate.

director: Added support for backend cluster "tags". This allows using a single director ring for multiple backend clusters. By default everything has an empty tag. A passdb lookup can return "director_tag" field containing the wanted tag name. If there aren't any backend servers with the wanted tag, it's treated the same as if there aren't any backend servers available (= wait for 30 secs for a backend and then return temporary failure). Tags can be added to configuration by adding @tag suffix to IPs/hosts. For example: director_mail_servers = 10.0.0.100-10.0.0.110@name1 10.0.0.120@name2 "doveadm director add" can also add tags either with @tag suffix or with -t parameter. "doveadm director status user@domain" requires giving the user's correct tag with -t parameter or the results won't be correct (empty tag's results are shown). Tags can't currently be changed for an existing host without removing it first.

director: If request is timed out, log an error.

Added initial implementation of a director process (for NFS users). There are still some unimplemented features and bugs. Also changing mail server list doesn't yet make sure that other directors won't assign the same user to a different server at the same time. --HG-- branch : HEAD