Handle potential race condition in search observer queue maintenance
In some cases, when e.g. objects are quicky deleted after updates, the
object might be already be deleted when being tried to trigger a
DELETE operation in the search queue. as a consequence, one can sometimes
observer errors like the one below. The new code double-checks now,
if the object for the DELETE operation still exists.
ERROR: insert or update on table "search_observer_queue" violates foreign key constraint "search_observer_queue_object_id_fk"
: DETAIL: Key (object_id)=(497509146) is not present in table "acs_objects".
: CONTEXT: SQL statement "insert into search_observer_queue (
