Small slots refactor (#2780)

* Deprecate SlotWorker::on_start

* start_slot_worker no longer needs an Arc

* start_slot_worker now always succeeds

* Removed on_exit parameter from start_*_worker

* Minor doc

* Fix node-template

substrate/node-template/src/service.rs

@@ -14,6 +14,7 @@ use substrate_service::{
 };
 use basic_authorship::ProposerFactory;
 use consensus::{import_queue, start_aura, AuraImportQueue, SlotDuration, NothingExtra};
+use futures::prelude::*;
 use substrate_client::{self as client, LongestChain};
 use primitives::{ed25519::Pair, Pair as PairT};
 use inherents::InherentDataProviders;
@@ -75,7 +76,7 @@ construct_service_factory! {
 					let client = service.client();
 					let select_chain = service.select_chain()
 						.ok_or_else(|| ServiceError::SelectChainRequired)?;
-					executor.spawn(start_aura(
+					let aura = start_aura(
 						SlotDuration::get_or_compute(&*client)?,
 						key.clone(),
 						client.clone(),
@@ -83,10 +84,10 @@ construct_service_factory! {
 						client,
 						proposer,
 						service.network(),
-						service.on_exit(),
 						service.config.custom.inherent_data_providers.clone(),
 						service.config.force_authoring,
-					)?);
+					)?;
+					executor.spawn(aura.select(service.on_exit()).then(|_| Ok(())));
 				}
 
 				Ok(service)