R1:6d5cde8e1cbc

Fix accessing private parent class properties in QueryFuture

Summary:
PHPStan complains about `Access to private property $exception of parent class Future` and `Access to private property $result of parent class Future` in `QueryFuture`.
Thus call the corresponding public functions provided by `Future` instead.

Test Plan: Run static code analysis; read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.