CamelIMAPXExtdServer: serialized handler registration, init fixup
CamelIMAPXExtdServer: serialized handler registration, init fixup

* made registration of untagged annotation handler registration
  thread-safe
* removed G_ONCE from handler registration in init(), so
  multiple instances of CamelIMAPXExtdServer can each
  register their handlers
* re-registering the same handler for the same server
  instance does currently occur and needs fixing

GNOME/evolution-kolab,GNOME/evolution-kolab