Collection.get_all_replicas needs proper implementation

The existing C bindings deliver just one document, so we currently raise a NotImplemented until correcting this.

The fix could be just properly getting the LCB_REPLICA_MODE_ALL properly passed into LCB, except that the value callback tries to make a dict keyed on the document key. So, even if the value callback does get called multiple times, it will overwrite the others. So probably that callback needs to be more clever as well.