Ceph and immutable & append only storage

The CEPH_OSD_ALLOC_HINT_FLAG_IMMUTABLE and CEPH_OSD_ALLOC_HINT_FLAG_APPEND_ONLY flags are set by rados bench via ALLOC_HINT_FLAG_IMMUTABLE and ALLOC_HINT_FLAG_APPEND_ONLY. It is used by BlueStore to decide on checksum and compression heuristics.