Add mechanism for caches to handle failure of the fast path on responses.

Add mechanism for caches to handle failure of the fast path on responses.

For now, responses have priority over requests (may want to revist this).

src/mem/cache/base_cache.cc:
src/mem/cache/base_cache.hh:
    Add mechanism for caches to handle failure of the fast path on responses.

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extra : convert_revision : 01524c727d1bb300cc21bdc989eb862ec8bf0b7a