Add module command for clearing performance measurements

The SmartRouter first sends a query to all targets and measures which one is faster. I cancels the other query when the first response has been received. The response time is stored, so that the next time a similar query arrives, it is sent to the faster target.
Currently it is not possible to explicitly erase the content of that performance "database", which makes it quite inconvenient to test the behaviour. A module command using which that database can be cleared is needed.