osd: load all classes on startup

This avoid creating a wide window between when ceph-osd is started and
when a request arrives needing a class and it is loaded.  In particular,
upgrading the packages in that window may cause linkage errors (if the
class API has changed, for example).

Fixes: #5752
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>