Support NOTIFY and change SOA serial

[buffrr]

The plugin should give a consistent SOA serial across multiple hsd instances.

I'm thinking the serial should be:

serial = floor(block height / 36)

So every zone version reflects a snapshot of the urkel tree. For example, if the block height = 90000, the SOA serial would be 2500.

Chain Reorgs problem:

If there is a reorg the exact same serial may give two different zones!! So the plugin should store the zone file and wait a few more blocks for confirmation BEFORE sending NOTIFY or updating SOA.

Example:

Block height = 90000
Current SOA serial = 2499, zone file stored 2499.zone (published)
Next SOA serial = 2500, zone file stored 2500.zone (inactive)

After maybe 12 confirmations/blocks

Block height = 90012
old SOA serial= 2499, zone file stored 2499.zone (removed)
Current SOA serial = 2500, zone file stored 2500.zone (published)

This solution requires the plugin to store two zones internally to keep SOA serial consistent and increasing order.