rgw/cache: This commit squashes the following commits for redis driver.

rgw/cache: This commit squashes the following commits for redis driver.

RGW: fix key_exists method for RedisDriver and clean up rgw_sal_d4n.cc
RGW: Implement RedisDriver::get_free_space
rgw/cache: modifying namespace from rgw::cal to rgw::cache.
RGW: Update D4N files to match CacheDriver changes
RGW: Fix D4N read workflow crashes
RGW: Update RedisDriver to match new CacheDriver structure; define set_attrs method
RGW: Switch out D4N cache methods with Redis driver methods
RGW: Update Cache Driver structure
RGW: Update cache files.
RGW: create redis cache driver files

Signed-off-by: Samarah <samarah.uriarte@ibm.com>