Change the import queue traits to take &mut self instead of &self (#3058)

* SharedFinalityProofRequestBuilder -> BoxFinalityProofRequestBuilder

* SharedThings -> BoxThings

* Fix tests

* build_request_data now takes &mut self

* The other traits now also take &mut self

* More or less fix tests

* Fix tests

* Fix more tests

* Moar tests

* Don't call make_block_import multiple time

* Fix doctest

substrate/test-utils/transaction-factory/src/lib.rs

@@ -176,5 +176,5 @@ fn import_block<F>(
 		auxiliary: Vec::new(),
 		fork_choice: ForkChoiceStrategy::LongestChain,
 	};
-	client.import_block(import, HashMap::new()).expect("Failed to import block");
+	(&**client).import_block(import, HashMap::new()).expect("Failed to import block");
 }