libsnapshot: Add function to verify merge sequence

This adds a function that verifies that the merge sequence does not
attempt to use blocks that have already been overwritten. It prints the
first such conflict to the logs.

Additionally, this adds a forwards merge iterator, and the ability to
ignore current progress in a merge for the purposes of verifying the
whole sequence.

This can be accessed via the VerifyMergeOps Function in Cow Reader, or
by using inspect_cow -v [COWFILE]

Include merged ops in inspect_cow listing with -a, and view ops in merge
order with -n

Bug: 200076590
Test: cow_api_test.InvalidMergeOrderTets
Change-Id: I893b9f8a8803cb6dd53225ec34224167b9fe2fda