Implement read_payload for the V1 protocol

Signed-off-by: John Else <john.else@citrix.com>
Switch the order of datasource and owner to match the plugin library.

Signed-off-by: John Else <john.else@citrix.com>
Remove redundant type definition

Signed-off-by: John Else <john.else@citrix.com>
Implement write_payload for the V1 protocol

Signed-off-by: John Else <john.else@citrix.com>
Verify the payload's checksum

Signed-off-by: John Else <john.else@citrix.com>