The DB.Update function takes a type of ACMETxt. However, the function
only requires the Value and Subdomain fields.
Refactor the function such that it takes ACMETxtPost instead of the full
ACMETxt record. This will simplify extraction of txt-record related
logic from the db code.
* Support for multiple TXT records per subdomain and database upgrade functionality
* Linter fixes
* Make sure the database upgrade routine works for PostgreSQL
* Move subdomain query outside of the upgrade transaction