zonefiles do not track canonical chain state correctly

[rafaelcr]

The zonefiles table has been returning incorrect results for BNS endpoint queries given that it does not keep the canonical state of the related transaction correctly.

There are lots of instances when an /attachments/new message from the node comes which includes two (or more) updates for the same zonefile but only one of them is actually canonical and valid. As it stands today, the zonefiles table is just UPDATEd with the last received value rather than the correct value.

Another related issue is that no validation is being done to prevent inserting a zonefile with the same hash more than once in the table, making it huge and very difficult to navigate around in event replays.