Move import lock onto backend (#2797)

* Drop import_lock from client, move it into backend, impl default version via static mutex

* still need to allow depcretion because of client.backend

* additional docs

* Remove default impl of get_import_lock, impl on instances

* Bump parking_lot to 0.8.0 accross the board

substrate/core/consensus/aura/Cargo.toml

@@ -23,7 +23,7 @@ authorities = { package = "substrate-consensus-authorities", path = "../authorit
 runtime_primitives = { package = "sr-primitives", path = "../../sr-primitives" }
 futures = "0.1.17"
 tokio = "0.1.7"
-parking_lot = "0.7.1"
+parking_lot = "0.8.0"
 log = "0.4"
 
 [dev-dependencies]

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

@@ -886,7 +886,6 @@ mod tests {
 			#[allow(deprecated)]
 			let select_chain = LongestChain::new(
 				client.backend().clone(),
-				client.import_lock().clone(),
 			);
 			let environ = Arc::new(DummyFactory(client.clone()));
 			import_notifications.push(