Also collect DNSSEC refresh signature statistics

Also collect DNSSEC refresh signature statistics

In addition to gather how many times signatures are created per
key in a zone, also count how many of those signature creations are
because of DNSSEC maintenance.  These maintenance counters are
incremented if a signature is refreshed (but the RRset did not
changed), when the DNSKEY RRset is changed, and when that leads
to additional RRset / RRSIG updates (for example SOA, NSEC).