Implement a "list" method for CAS

[wlandau]

It's complicated to manually write logic for maintaining an in-memory cache of hashes that exist in a CAS system. A new "list" method would avoid having to do this inside the "exists" method.

[wlandau]

On second thought, this is not feasible because each CAS system is defined at the level of each target.

[wlandau]

Maybe targets can maintain multiple lists of keys on a CAS-by-CAS basis?