NDLCommons feature - reset metadata

[paul-ruth]

We need a function in NDLCommons that allows AHAB (or others) to reset the "metadata" describing an element in a modify request. This would be necessary when AHAB is being used by indecisive users who add/remove some elements and undo those adds before a commit.

[ibaldin]

Metadata is membership of elements in reservation list (for new requests) and modify elements in modify requests.

Reset may not be the right approach though - not clear what should happen to the elements in this case.