Add v3 writer

Adding v3 writer that works off of Cow Operation v3. Adding test file
that will test this new writer. Adding in stub implementations to v3 writer. None of these functions
have to work yet, we just need the implementations here to compile.

Test: m libsnapshot
Change-Id: If86437d5ceb2c33520d4ca26dea5193984f86546
4 files changed