libsnapshot: implement resume buffer

Add resume space to cow v3. Resume buffer goes after header and scratch
space, and is currently set to contain 4 resume points. When AddLabel is
called, the oldest label is replaced with newest one.

Parser will parse up until the last resumable op from a given label.

Test: cow_api_test
Change-Id: Ie072f245721776887d59c96dad296965ad31a5cc