make policy an interface

[tushar-zomato]

add option to always admit new items

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[joshua-goldstein]

There is the option of setting MaxCost and NumCounters to be very large values, so that the cache will admit essentially all items (except those dropped in the buffers). You might also be able to add items with the cost of each item to be 0. These are much easier solutions than adding an entirely new policy.

[MichelDiz]

Why this renaming?