osdc/Objecter: more explicit session handling

Wrap all ref counting on sessions relating
to their objects insign assign/remove fns.

Use put_session and get_session everywhere
and log from these fns.

Assert op maps are empty in OSDSession destructor
as the entries in these maps should always come
with a reference, and if the maps are still populated
we are leaking a reference.

Signed-off-by: John Spray <john.spray@redhat.com>