[client-connection] explicitly check that the client-connection is open
The check is also perform by the _srv_cnx property. But we do not need those
property anymore.
Latest changes
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* table of `all latest changes`_
Links below is providing useful RQL query example.
.. _all latest changes: view?rql=Any+M%2CX+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30