libsnapshot: sync header metadata

After we write emit a label, we need to update the number of resume
points + sequence data and op_count. Realistically we could just call
Finalize, but maybe synching these specific fields could prevent
unexpected outcomes.

Test: cow_api_test
Change-Id: I1585601a134221689ce8d5675a2a3e32f1e8a0e6