Executable queue for ancient blocks inclusion (#208)

* Executable queue for ancient blocks inclusion
* Add drop trait for client
* Added shutdown to tests
* Remove doubled call
* Use reth-util from reth repo

crates/ethcore/service/src/service.rs

@@ -142,6 +142,7 @@ impl ClientService {
     pub fn shutdown(&self) {
         trace!(target: "shutdown", "Shutting down Client Service");
         self.snapshot.shutdown();
+        self.client.shutdown();
     }
 }