Update records only if they have been changed

[JonasGroeger]

Prior, for every zone change, all records in the database were
updated. This patch improves upon that so that only changed records
in a zone are updated in the database.

The zone comment is still unchecked. A new serial will be generated,
even if there is no record change.

This fixes #234, #166.

[JonasGroeger]

@eg Any chance this will get reviewed?

[edmondas]

Thanks.

[RavenB]

Can we still have the option to do a full zone refresh ? This seems no longer be possible.
Although the update is nice, sometimes we might wish to have a full refresh. (ie can this be corrected to allow us to choose ?) or have a an option somewhere to have full refresh. (without needing to to go the pdns_tools itself.) Or does a full refresh still occur ?

[JonasGroeger]

@RavenB What exactly do you mean with "full refresh"?

[edmondas]

Zone transfer is still possible, because serial number is updated even there are no changes in records.
But situation changes if you use 0 for serial number and according to PowerDNS documentation:

If left at 0, the default, PDNS will perform an internal list of the domain to determine highest change_date field of all records within the zone, and use that as the zone serial number. This means that the serial number is always raised when changes are made to the zone, as long as the change_date field is being set.

So after @JonasGroeger changes you will not be able to do "full refresh".