main: fix bug where thread pool isn't joined

Pointed out here:
https://github.com/rust-analyzer/rust-analyzer/pull/4133#issuecomment-619386272

crates/rust-analyzer/src/main_loop.rs

@@ -222,6 +222,7 @@ pub fn main_loop(ws_roots: Vec<PathBuf>, config: Config, connection: Connection)
     libdata_receiver.into_iter().for_each(drop);
     log::info!("...tasks have finished");
     log::info!("joining threadpool...");
+    pool.join();
     drop(pool);
     log::info!("...threadpool has finished");