cql_test_env: optimize handling result_message::exception
cql_test_env: optimize handling result_message::exception

The single_node_cql_env uses query_processor::execute_xyz family of
methods to perform operations. Due to previous commits in this series,
they allocate one more task than before - a continuation that converts
result_message::exception into an exceptional future. We can recover
that one task by using variants of those methods which do not perform a
conversion, and turn .finally() invocations into .then()s which perform
conversion manually.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla