Fix various minor bugs with ExecFuture

Summary:

• Fix a bug where write('') would hang for 1s before closing stdin.
• Fix a bug where start() does not return $this.
• Fix a bug where setStdinSizeLimit() does not return $this.
• Increase ExecFuture timeout precision.

Test Plan: Added unit tests, ran unit tests.

Reviewers: btrahan, aran

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1868