Refactor off COW header v3

Cow reader will store header v3 in memory. ReadCowHeader can remain
mostly unchanged since the cow prefix is the same.
header->prefix.header_size will then tell us if we're reading a v3 or v2
header. v3 header is strictly a superset of v2 header so we can read a
v2 header into a v3 struct.

Added a test case to test_v3 where we write a header using v2_writer and
ensure CowReader is able to read it.

Test: cow_api_test
Change-Id: I142f18d871322930b7dc341c342c8b63a481341c