Refactor: Use Go Generics for RecordConfig processing

[tlimoncelli]

• Use Generics to reduce code repetition
• Split out the parser for each RecordType so they can be used in multiple places
• Make CF_SINGLE_REDIRECT more like the other RecorTypes
• Create MustImportFromLegacy() for use in testing.
• More error checking
• Replace PopulateFields*() with RecordUpdateFields()
• RegisterType() -> MustRegisterType()
• Add MustCreateRecord() for unit tests
• Fix pkg/js/parse_tests/007-importTransformTTL.json (expected result was wrong!)
• Change SetTarget() to SetTargetA() where possible