Crash during OTA merge may lead to blocks with stale data

This is a corner case wherein a crash during OTA
merge can lead to missing of some COW operations to be
merged thereby some blocks may end up with stale data.

Fix here is to avoid any re-ordering of COW operations.
Merge the COW operations as present in the COW file.

New tests have been added to cow_snapuserd.

Bug: 194955361
Test: cow_snapuserd_test, Incremental OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Id895fe7a3d6b4510676490a86d0caf62dec9b079