cubicweb #2500154 Refactor dbapi.connect [open]

Since zmq backend implementation, appid has no evident meaning anymore (it still had with pyro, within the nameserver).

The proposed refactoring is to use a url with a custom protocol to encode all the information needed to connect to a listening Cubicweb server, something like : cw://$instancename zmq://$zmqaddress pyro://$pyroinfos

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