libsnapshot: Add cluster breaks after ops

Previously, we'd check if a new cluster was needed before we added a Cow
Operation. This would cause an op's associated data to go to the wrong
location, so instead we check if we'll need a new cluster after writing
each op.

Bug: 172026020
Test: cow_api_test (ClusterCompressGz)
Change-Id: Ia43afedcfd430961b34f5914da4265b89e6fadb9