Stop authoring blocks when offline (#1655)

* Don't author blocks when offline

* Increased canonicalization delay

* Fixed test

substrate/core/consensus/common/src/lib.rs

@@ -88,6 +88,9 @@ pub trait SyncOracle {
 	/// Whether the synchronization service is undergoing major sync.
 	/// Returns true if so.
 	fn is_major_syncing(&self) -> bool;
+	/// Whether the synchronization service is offline.
+	/// Returns true if so.
+	fn is_offline(&self) -> bool;
 }
 
 /// A synchronization oracle for when there is no network.
@@ -96,10 +99,14 @@ pub struct NoNetwork;
 
 impl SyncOracle for NoNetwork {
 	fn is_major_syncing(&self) -> bool { false }
+	fn is_offline(&self) -> bool { false }
 }
 
 impl<T: SyncOracle> SyncOracle for Arc<T> {
 	fn is_major_syncing(&self) -> bool {
 		T::is_major_syncing(&*self)
 	}
+	fn is_offline(&self) -> bool {
+		T::is_offline(&*self)
+	}
 }