rgw/d4n: squashing commits related to handling of

rgw/d4n: squashing commits related to handling of
clean (non-dirty) objects for delete_obj method.

1. rgw/d4n: miscellaneous fixes that include modifying
set_head_obj_dir_entry() and delete_obj method to handle
clean objects only.
2. rgw/d4n: changes to design when write cache is turned off:
a. do not store a hash entry for latest version for versioned
buckets, the latest version will always be fetched from backend store,
to maintain correct order of versions during a delete object request.

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