rgw/cache: this commit squashes following commits related to changes in

rgw/cache: this commit squashes following commits related to changes in
redis driver and ssd driver.

rgw/redis: Add `del`, `put_async`, and `calculate_free_space` methods to RedisDriver
rgw/redis: Work on RedisDriver `put_async` and `get_async` methods
rgw/cache: Add error-checking to `SSDDriver::set_attr`
rgw/redis: Remove `calculate_free_space` method from RedisDriver, fix
`free_space` calculations, clean up async code

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