objecter: split objecter initialization

objecter: split objecter initialization

Separate objecter initialization to non cluster related work (e.g.,
internal data structures, other registrations), and to operations that
can initiate cluster interaction. This is so that we don't hit a rare
race where we can get called indirectly from one of the dispatcher callbacks
e.g., into handle_osd_map() when not yet being initialized.
This requires that objecter->init() should be called before
messenger->add_dispatcher_head(), and objecter->start() after it.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>