dnssec SERIAL handling
The SERIAL handling is DNSSEC signed zones is little bit awkward at the moment.
We need to be able to select the SERIAL handling (default and per zone) from:
a) increment b) unixtime c) no touching
possibly with some form of autodetection, so this won't happen:
2013-11-18T16:02:14 [warning] Zone file for 'sury.cz.' changed, but serial didn't - won't create changesets.
The default should be 'auto' that would default to unixtime unless the current SERIAL is bigger and in form of YYYYMMDDXX in which case it should fall back to increment.
Also in case the user forgets to bump the SERIAL (in not-yet-flushed zone), but the zone needs resigning we should increment starting with the current SERIAL in the journal and not the value in the zone file. That is if the SERIAL hasn't changed from last time.