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/client/db/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["Parity Technologies <admin@parity.io>"]
 edition = "2018"
 
 [dependencies]
-parking_lot = "0.7.1"
+parking_lot = "0.8"
 log = "0.4"
 kvdb = { git = "https://github.com/paritytech/parity-common", rev="b0317f649ab2c665b7987b8475878fc4d2e1f81d" }
 # FIXME replace with release as soon as our rocksdb changes are released upstream https://github.com/paritytech/parity-common/issues/88

substrate/core/client/db/src/lib.rs

@@ -656,6 +656,7 @@ pub struct Backend<Block: BlockT> {
 	blockchain: BlockchainDb<Block>,
 	canonicalization_delay: u64,
 	shared_cache: SharedCache<Block, Blake2Hasher>,
+	import_lock: Mutex<()>,
 }
 
 impl<Block: BlockT<Hash=H256>> Backend<Block> {
@@ -722,6 +723,7 @@ impl<Block: BlockT<Hash=H256>> Backend<Block> {
 			blockchain,
 			canonicalization_delay,
 			shared_cache: new_shared_cache(state_cache_size),
+			import_lock: Default::default(),
 		})
 	}
 
@@ -1350,6 +1352,10 @@ impl<Block> client::backend::Backend<Block, Blake2Hasher> for Backend<Block> whe
 		}
 		Ok(())
 	}
+
+	fn get_import_lock(&self) -> &Mutex<()> {
+		&self.import_lock
+	}
 }
 
 impl<Block> client::backend::LocalBackend<Block, Blake2Hasher> for Backend<Block>