Decouple BABE from session (#3760)

substrate/node/runtime/src/lib.rs

@@ -84,7 +84,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to equal spec_version. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 170,
+	spec_version: 171,
 	impl_version: 171,
 	apis: RUNTIME_API_VERSIONS,
 };
@@ -142,6 +142,7 @@ parameter_types! {
 impl babe::Trait for Runtime {
 	type EpochDuration = EpochDuration;
 	type ExpectedBlockTime = ExpectedBlockTime;
+	type EpochChangeTrigger = babe::ExternalTrigger;
 }
 
 impl indices::Trait for Runtime {