Support timeouts for Phage commands

Summary: Ref T2794. Occasionally stuff hangs for whatever reason. Provide a crude per-command timeout as a general tool for dealing with this.

Test Plan: Added some sleep and ran with --timeout, saw commands get killed and reported as failed.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2794

Differential Revision: https://secure.phabricator.com/D17463