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/node/cli/src/service.rs

@@ -205,10 +205,7 @@ construct_service_factory! {
 		SelectChain = LongestChain<FullBackend<Self>, Self::Block>
 			{ |config: &FactoryFullConfiguration<Self>, client: Arc<FullClient<Self>>| {
 				#[allow(deprecated)]
-				Ok(LongestChain::new(
-					client.backend().clone(),
-					client.import_lock()
-				))
+				Ok(LongestChain::new(client.backend().clone()))
 			}
 		},
 		FinalityProofProvider = { |client: Arc<FullClient<Self>>| {