vinyl: fix crash on scheduler stop

If there's a dropped index referenced by a pending task on scheduler
stop, vy_scheduler_stop() will crash while trying to delete it:

  vy_scheduler_stop
   vy_task_delete
    vy_index_unref
     vy_index_delete
      vy_range_tree_free_cb
       vy_scheduler_remove_range

because there will be a range which is not on the dump/compact heap.

Fix that by completing all processed tasks and aborting (i.e. setting
status to -1 and calling ->complete) all pending tasks when stopping the
scheduler.