Add CowOperationV2

this is going to replace the current version of CowOperation and will
work with writer v2 and parser v2. This will be the on disk format of
the cow, while CowOperation will be updated to be the in memory format
of 15 bytes (implicitly will be the v3 version).

Test: m libsnapshot

Test: m libsnapshot
Change-Id: Ibd00ef014a9fc11cdf2bad97c52462db8eef9502