libsnapshot: Add CowRevMergeOpIter

This presents cow ops in the reverse of their merge order, as requested
when we perform a merge. It starts with sorted non-ordered ops sorted by
blocks, and then the ordered ops. Already merged ops are removed.

Bug: 177104308
Test: cow_api_test
Change-Id: I864bcab5cda447cf4a197abc02d954f9db08b744