cqlsh: show partial trace if incomplete after max_trace_wait

If a trace hasn't completed within max_trace_wait, cqlsh will say the trace is unavailable and not show anything. It (and the underlying python driver) determines when the trace is complete by checking if the duration column in system_traces.sessions is non-null. If duration is null but we still have some trace events when the timeout is hit, cqlsh should print whatever trace events we have along with a warning about it being incomplete.