libsnapshot:snapuserd: Read the cow_operation in reverse order

This is required for merge IO path. During merge process,
kernel scans the metadata page from backwards when merge
is initiated. Since, merge ordering should follow our internal
COW format, read the COW operations from backwards and populate the metadata.

Additionally, Kernel can merge successive operations if the two chunk IDs
are contiguous. This can be problematic when there is a crash
during merge; specifically when the merge operation has dependency.
These dependencies can only happen during copy operations of the
overlapping regions. To avoid this problem, we make sure that
copy operations are merged individually.

Test: cow_snapuserd_test
Bug: 168311203

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ic9c52b323ce58af897cfa343bf9277c8f1f022e3