Make the "result" property on Future private

Summary: Depends on D21033. Ref T11968. This is just an incremental step in modernizing Future and making it more robust. Currently, subclasses are expected to write directly to $this->result, but this isn't consistent with how modern classes generally work.

Test Plan: Ran unit tests, created this revision.

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21034