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.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25755