rgw: this commit squashes commits performing cleanup in various files

rgw: this commit squashes commits performing cleanup in various files
like d4n filter, policy, cache backend, aio and process_env.

d4n/filter: Remove unnecessary erasure of attrs in read op's `prepare`
method
d4n: Remove `dout` definitions from header files
rgw: Remove unnecessary includes in `rgw_process_env.h`
rgw: Remove `cache_read_op` from `rgw_aio.h`
rgw: Return POSIX error codes and perform minor cleanup for D4N and
Redis files

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