V3 writer header

V3 writer set up header. Anything related to cluster ops can be set to
zero, as V3 cow format will no longer support this. All footer related
code can also be dropped as the footer isn't being used.

Test: cow_api_test
Change-Id: I39ece8eaa0813c5991d53b5e0ff9271f27e0662a
2 files changed