osdc/Objecter: fix handling for osd_command dne/down cases

Generalize the map check machinery that the pool dne check uses to also
get the latest map for OSD down/dne checks.  This is better semantics, but
more important fixes the more immediate bug of returning the error code
to the caller from the osd_command -> _submit_command (that is ignored by
pretty much any caller) and then never triggering the callback.

Signed-off-by: Sage Weil <sage@inktank.com>