new statistic: number of addresses assigned

There is a customer that is interested in observing how many leases were assigned during the day.

This is different than subnet[id].assigned-addresses, which shows the current number of assigned addresses. This stat is going up and down.

To address the customer's need we should have a cumulative counter that counts each new allocation.

Incidentally, this may also be useful for Stork.

For details, see support #15039