* BabeWorker -> BabeSlotWorker
* SlotWorker::notify_slot: similar to claim_slot, but called no matter authoring
* Wrap the future with a new struct BabeWorker
* Add type definition slot_notification_sinks
* Function slot_notification_streams for the receiver side
* Get a handle of slot_notification_sinks in BabeSlotWorker
* Implement notify_slot
* Switch to use bounded mpsc
* Do not drop the sink when channel is full
Only skip sending the message and emit a warning, because it is recoverable.
* Fix future type bounds
* Add must_use and sink type alias