rgw/d4n: this commit squashes the following commits related to

rgw/d4n: this commit squashes the following commits related to
clean up and removal of cpp_redis.

d4n/filter: Add `optional_yield` to CacheDriver calls in D4N Filter and
make minor updates to several filter methods
rgw/d4n: fix compilation issue.
rgw/cache: Add `del` method to CacheDriver and SSDDriver
cmake/d4n: Remove unnecessary D4N lines
rgw: Add `io_context` to D4N Filter and RedisDriver, remove `cpp_redis`
library from RedisDriver, and perform minor cleanup
d4n: Remove `cpp_redis` library from D4N directory and policy; update calls in filter; move Entry struct to base CachePolicy class
build/cpp_redis: Remove `cpp_redis` library
rgw/d4n: including <boost/asio/detached.hpp> wheerever needed.
rgw/d4n : fixes to d4n filter, policy, directory and redis driver files
for compilation errors.

Signed-off-by: Samarah <samarah.uriarte@ibm.com>
Signed-off-by: Casey Bodley <cbodley@redhat.com>
Co-authored-by: Pritha Srivastava <prsrivas@redhat.com>