librados: wait for osdmap for commands that need it

In commit 7e1cf87b5158c870e2a118ed6d316be8cb9818ce we stopped waiting for
the osdmap on start because the Objecter will normally wait, but for some
commands we assume the osdmap is recent(ish).

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>