Remote transaction execution (#4684)

* return errors on database corruption

* fix tests, json tests

* fix remainder of build

* buffer flow -> request credits

* proving state backend

* generate transaction proofs from provider

* network messages for transaction proof

* transaction proof test

* test for transaction proof message

* fix call bug

* request transaction proofs from on_demand

* most of proved_execution rpc

* proved execution future

util/src/hashdb.rs

@@ -125,3 +125,13 @@ impl<T: HashDB> AsHashDB for T {
 		self
 	}
 }
+
+impl<'a> AsHashDB for &'a mut HashDB {
+	fn as_hashdb(&self) -> &HashDB {
+		&**self
+	}
+
+	fn as_hashdb_mut(&mut self) -> &mut HashDB {
+		&mut **self
+	}
+}