DEBUG: counters: make COUNT_IF() only appear at DEBUG_COUNTERS>=1

DEBUG: counters: make COUNT_IF() only appear at DEBUG_COUNTERS>=1

COUNT_IF() is convenient but can be heavy since some of them were found
to trigger often (roughly 1 counter per request on avg). This might even
have an impact on large setups due to the cost of a shared cache line
bouncing between multiple cores. For now there's no way to disable it,
so let's only enable it when DEBUG_COUNTERS is 1 or above. A future
change will make it configurable.