libsnapshot: Only sync after labels

This changes labels to belong at the end of the set of ops that they
refer to. We only sync after writing a label, or the footer, saving the
cost of syncing after ever op.

Change-Id: Iee9dd69132b8e3321eccfe1e43fa0c072a94d3bd
Bug: 172026020
Test: cow_api_test
4 files changed