rgw/d4n: squashing the following commits for restoring

rgw/d4n: squashing the following commits for restoring
in-memory data for LFUDA policy and dirty objects from
cache on disk

1. rgw/d4n: restore in memory data structure for dirty
objects using the xattrs in the head block of an object
in the cache.
2. rgw/d4n: replacing string cache attr names with constexpr char.
3. rgw/d4n: restore LFUDA policy data from cache on disk.
4. rgw/d4n: correcting the key used while updating the LFUDA data structure
for the head object block in the read path.

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