Switch up Cow Format to be resumable

This switches up the format to alternate ops with data, followed by a
footer containing additional meta information. This allows the file to
be resumed at arbitrary points if writing gets interrupted by power
loss.
Also adds a label op, which allows labeling future ops as connected.
If the footer is missing, Append will treat the last label as possibly
corrupt, and ignore it.

Change-Id: I126e15837d710776f9396e7afc9b0cd595e26b59
Bug: 168829493
Test: cow_api_test