Split _wait and _unref

This also removes BackupProvider::join in favour of implementing
Future directly.  I wondered about implementing a FusedFutre to make
this a little safer but it would introduce a dependency on the futures
crate in deltachat-ffi which did not exist yet, so I didn't do that.

deltachat-jsonrpc/src/api/mod.rs

@@ -1371,7 +1371,7 @@ impl CommandApi {
                 return Err(err);
             }
         };
-        let res = provider.join().await;
+        let res = provider.await;
         ctx.start_io().await;
         res
     }