celery: msgpack serialization usage
celery: msgpack serialization usage

* Changes default celery configuration to use msgpack instead of
  pickle and use rabbitmq instead of redis broker, after some
  problems under heavy load.

Tested-by: Jan Aage Lavik <8d55e006a1b29770f275a5c59a7341f17f8e69d5@cern.ch>

tiborsimko/invenio-celery,tiborsimko/invenio-celery,inveniosoftware/invenio-celery,inveniosoftware/invenio-celery