scanner: print throughput statistics over a 15-seconds window (#146)
scanner: print throughput statistics over a 15-seconds window (#146)

A big initial buffer might skew the statistic to the point of making it
useless for the whole run.  Also, this gives fast feedback on the speed
of the system.  15s is arbitrary.

Also took a note about the statistics goroutine not being cleaned up.
google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go