fix(scan operation thread): fix creating a cql session
fix(scan operation thread): fix creating a cql session

	Fix usage and calls of CQL connections for the scan-operation thread.
	This is in order to better cleanup used connections,
	and avoid encountering issues like:
	https://github.com/scylladb/scylla-bench/issues/95

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests