cubicweb #1922945 conditional log of rql queries [open]

Make it possible to configure a threshold (a time in milliseconds) for which when a RQL query takes longer than this value, it is logged down.

The expected logged data should include the full RQL query, the total amount of time it took to return, and probably some other contextual informations.

This config should be set per source; we do want longer threshold for distant CW sources than for the system one.