Any VLOG statement including debug ones in OVS will always invoke vlog_valist()

https://github.com/openvswitch/ovs/blob/master/lib/vlog.c#L1093

This function will ALWAYS lock the logging file mutex at entry regardless of the fact if it:

1. Need to log at all
2. Is logging to file

It is trivial to modify it so that it fast-paths across the "need to log" criteria and mutex-locks only if it is actually supposed to log.

This is invoked from > 2000 places including locations which are in performance critical/time critical sections.

In addition to being a performance issue, this is also an issue when trying to improve performance by splitting work between threads because it makes the codebase highly Heisenbuggy - multithreaded code works with debug on for a particular module and stops working without it.