registry: Create per record+owner metadata records (TXT Registry)

[mikenairn]

What

Update the TXT registry to create a registry entry per owner/record combination instead of a single record entry that is updated with all owners.

Pros:

• Each DNSRecord will only ever be updating it's own TXT records avoiding all possible conflicts on TXT record updates between records/clusters updating the same record set.
• Removes the limit of ~20 records max for the same dns name (rootHost) . Upper limit would only be limited by the number of records allowed in a zone.
• Would resolve a lot of issues we hit when deleting records Improve DNSRecord deletion to better ensure changes are persisted  #181.
• More possibility for expanding the metadata saved in these records. Health, heartbeat etc..
• Is more like how you would do it if we were talking to a real database or cache i.e. redis.

Cons:

• More TXT records, one per owner on every record that is shared.

Considerations

• Need to migrate from the old format to the new

[maleck13]

This sounds like the ideal approach to me where the we remove more shared records and increase the upper limit. Any thoughts on how the migration would happen? I assume there would be no impact on existing non txt records right?
I think we should prioritise this change.