Benchmark overhead of session_trace

Knowing how much CPU and memory overhead there is to enabling session_trace would make it easier to evaluate how reasonable it would be to automatically turn it on when unexpected situations occur that could be caused due to a bug in MaxScale.

With the recent addition of session_trace_match, this becomes even more important as it is the only way to debug some problems in production environments where the overhead of enabling log_info is unacceptable. Knowing in advance how much this tracing costs would also inform us of how to possibly improve it so that an in-memory log could always be on by default.