Move statistics calculation from readwritesplit into the core

Readwritesplit calculates a number of metrics that would be of general use.

The average activity percentage of a session currently counts the time that the session is alive and compares it to how much of the time the client is active. This can be more accurately measured by the protocol layer as queuing internally in MaxScale distorts the measurement.

The average number of queries per session is also something that's of use which can be moved into the core.