libsnapshot: Switch merge to CowRevMergeOpItr

This switches merge code over from using the old RevOpItr to the new
MergeRevOpItr. Since there are no other users, RevOpItr is no longer
needed.

Changed names of copy_ops_ and total_data_ops_ to more accurately
reflect their meaning.

Bug: 177104308
Test: cow_snapuserd_test
Change-Id: Ic053be4877cfdc86656551f5a3d5d95f3825f937