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/cli/src/service.rs

@@ -26,6 +26,7 @@ use consensus::{import_queue, start_aura, AuraImportQueue, SlotDuration, Nothing
 use grandpa::{self, FinalityProofProvider as GrandpaFinalityProofProvider};
 use node_executor;
 use primitives::{Pair as PairT, ed25519};
+use futures::prelude::*;
 use node_primitives::Block;
 use node_runtime::{GenesisConfig, RuntimeApi};
 use substrate_service::{
@@ -92,7 +93,7 @@ construct_service_factory! {
 					let client = service.client();
 					let select_chain = service.select_chain()
 						.ok_or(ServiceError::SelectChainRequired)?;
-					executor.spawn(start_aura(
+					let aura = start_aura(
 						SlotDuration::get_or_compute(&*client)?,
 						key.clone(),
 						client,
@@ -100,10 +101,10 @@ construct_service_factory! {
 						block_import.clone(),
 						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(())));
 
 					info!("Running Grandpa session as Authority {}", key.public());
 				}