rgw/cache: this commit removes indexing from ssd and redis

rgw/cache: this commit removes indexing from ssd and redis
driver and squashes the following commits.

rgw/cache: modifications to the ssd cache driver after removing
indexing from the cache backend.
rgw/cache: modifications to redis cache driver, after
removing indexing from cache backend.
rgw/cache: modifications to policy driver code, commenting out
some pieces which refer to cache apis. Those apis have been
removed as part of modification to remove indexing from the
cache backend. These need to appropriately re-instated, once
we finalize which module will contain the data structures
related to cache policy.

Signed-off-by: Pritha Srivastava <prsrivas@redhat.com>