CamelIMAPXExtdServer: register untagged handlers once per server instance
CamelIMAPXExtdServer: register untagged handlers once per server instance

* make sure we do register the untagged handler functions
  only once per CamelIMAPXServer instance
* using a G_ONCE here would not work, since there would
  be handler registration only once per process (not
  once per server instance), which leads to arbitrary
  folders not being down-synced (since for one or the
  other server instance, there's no untagged handler
  function (metadata, ACL) registered)

GNOME/evolution-kolab,GNOME/evolution-kolab