don't deadlock on shutdown

Specifically, when we tear down IO threads, we should take care to
dispose connection.

closes #1775

crates/ra_lsp_server/src/main.rs

@@ -66,7 +66,7 @@ fn run_server() -> Result<()> {
         workspace_roots,
         initialize_params.capabilities,
         server_config,
-        &connection,
+        connection,
     )?;
 
     log::info!("shutting down IO...");