[MiniDNS] Update Storage layer to return objects rather than simple dictionaries

Update Storage layer to return objects rather than simple dictionaries

For MiniDNS - This allows us to build out structured RData objects.

This has advantages beyond MiniDNS, for example, validation rules can be
segregated into isolated classes for each object.

This step will create the following objects, have Storage return them, and
update the remaining code to handle these changes:

* Quota
* Server
* TLD
* TsigKey
* Tenant[1]
* Domain
* RecordSet
* Record
* Blacklist

[1]: Tenant is very much an "odd one out", and is only used in a V1 admin API extension.