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 <jan.age.lavik@cern.ch>
search: change union_update to union

* Due to changes in the behaviour of intbitset union_update function,
  union should be used if a return result is expected.

Reported-by: Kamil Neczaj <kamil.neczaj@cern.ch>
Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>