Update SOA record on zone change

[bartoszpijet]

Hi, Once the zone is being updated SOA record stays the same.
I think that it should increase its serial, otherwise, when you use PDNS in master/slave setting, the slave will not update zone records after receiving notify command (at least some records will stay the same from my experience).

I'll try to implement this functionality as a function in dnsManager.py that will trigger any DNS change and note my progress here.

[bartoszpijet]

Created pull request on branch 2.1.2-dev #768