Implement Default for MDAHeader
Implement Default for MDAHeader

This should make an MDAHeader which has all 0s for every field.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

trgill/stratisd,mulkieran/stratisd,mulkieran/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,trgill/stratisd