libsnapshot: Add IsOrderedOp

Ordered Ops are Cow Ops with source block dependencies.
Currently this is only Copy Ops, but will extend to XOR
blocks when they're added.

Bug: 177104308
Test: Builds, does not impace cow_api_test/cow_snapuserd_test
Change-Id: I0a50dfe3316ec3a3b1d035335f094c62296649a5