By using the cumulative time since the zone expiration (instead of the last
time interval) we achieve an approximation of exponential retry backoff with
base 2 (rather than with base 3, as it was until now). Now, with every retry attempt,
the interval between refreshes only doubles (with up to 30 second jitter).

This change helps avoid excessive waiting for a refresh when a few early attempts
fail.

kzonesign: implemented zone verify

See merge request !1199

Geoip checks

Closes #778

See merge request !1407

Rrsig-refresh2

See merge request !1404

tests: fail if some test segfaults or killed

See merge request !1406

Replace defective conf_mix_iter() with conf_mix_iter_init()

See merge request !1405

Unreachable remotes

Closes #383

See merge request !1395

Remote groups

See merge request !1403

dnssec: rrsig-refresh: warn if too low

See merge request !1402

DS push: use DNSKEY TTL for parent DS

See merge request !1401

NSEC3 iterations defaults to 0

See merge request !1400

tests: scenario of alg roll with offline KSK

See merge request !1399