Merge remote-tracking branch 'parity/split-internal-seal' into auth-round

ethcore/src/engines/mod.rs

@@ -73,6 +73,8 @@ pub trait Engine : Sync + Send {
 	/// Block transformation functions, after the transactions.
 	fn on_close_block(&self, _block: &mut ExecutedBlock) {}
 
+	/// If true, generate_seal has to be implemented.
+	fn seals_internally(&self) -> bool { false }
 	/// Attempt to seal the block internally.
 	///
 	/// If `Some` is returned, then you get a valid seal.