[repo] Add a publish/subscribe mechanism for inter-instance communication using zmq Each repo can have a publishing and any number of subscribing sockets whose addresses are specified in the instance's configuration. An application or cube can subscribe to some 'topics', and give a callback that gets called when a message matching that topic is received. As a proof of concept, this introduces a hook to clean up the caches associated with the repository when an entity is deleted. A subscription is added using Repository::zmq::add_subscription; the callback receives a list representing the received multi-part message as argument (the first element of the message is its topic).

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