osd: promotion on 2nd read for cache tiering

Signed-off-by: Zhiqiang Wang <wonzhq@hotmail.com>
ceph_test_rados_api_tier: test promote-on-second-read behavior

Signed-off-by: Zhiqiang Wang <wonzhq@hotmail.com>
Signed-off-by: Sage Weil <sage@redhat.com>
Merge branch 'wip-cache-second'

Reviewed-by: Samuel Just <sam.just@inktank.com>