OAuthServer - implement destructible interface on oauth server client objects

Summary: Fixes T6955.

Test Plan: made an oauth app. made a test authorization. ran bin/remove destroy <phid of oauth client> and there were no errors. verified oauth app and test authorization were both gone.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6955

Differential Revision: https://secure.phabricator.com/D11378