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>
(cherry picked from commit c24e652d8c5e693498814ebe38c6adbec079ea36)

Conflicts:
	src/osd/ClassHandler.cc
osd: make open classes on start optional

This is cuttlefish; default to the old behavior!

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 6f996223fb34650771772b88355046746f238cf2)