Statistics on average sigs/sec not accurate

The statistics on number of signatures per second include the time to check if RRsets need a new signature. This results in a lower value of signatures per second when only few signatures need to be (re)created, because more time is spend on checking.

For example, with the nl zone we see that there is a baseline of 25 seconds to go through the zone and one second more for every +- 13.000 created signatures. In other words: Creating 26.000 signatures takes 27 seconds: about 963 sigs/sec.