Remove the Priority Field in V2

The priority field is now the first item in the data field for
MX and SRV records, instead of beng a separate field.
V1 has not been changed and remains a separate
field.

Change-Id: Ie367b796b1a501d8a78e98d690a4b482dcabe8da
Implements: blueprint remove-priority-field
Merge "Remove the Priority Field in V2"