Fix possible OnceCell panic in Proxy::connection() (#186)

* Fix possible OnceCell panic in Proxy::connection()

When multiple tasks are trying to create proxies simultaneously,
another task may initialize the zbus-Connection-OnceCell while
the connection is initialized.

This ensures that only the first task to establish a connection
initializes the OnceCell

* fix rust-fmt suggestion

src/proxy.rs

@@ -38,8 +38,8 @@ impl<'a> Proxy<'a> {
             Ok(cnx.clone())
         } else {
             let cnx = zbus::Connection::session().await?;
-            SESSION.set(cnx.clone()).expect("Can't reset a OnceCell");
-            Ok(cnx)
+            // during `await` another task may have initialized the cell
+            Ok(SESSION.get_or_init(|| cnx).clone())
         }
     }