RPC api for offchain storage (#4694)

* Rpc api for offchain storage

* Replace Vec<u8> to Bytes, replace Mutex to RwLock

* Remove pub

* Modify copyright year

substrate/client/rpc/src/lib.rs

@@ -28,5 +28,6 @@ pub use rpc::IoHandlerExtension as RpcExtension;
 
 pub mod author;
 pub mod chain;
+pub mod offchain;
 pub mod state;
 pub mod system;