mon: add 'pg dump delta' to get just the rate info

Still include it in the basic 'pg dump summary' info.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #524 from ceph/wip-mon-delta

mon: add 'pg dump delta' to get just the rate info

Reviewed-by: Samuel Just <sam.just@inktank.com>