Make Phage agents stream output continuously

Summary:
Ref T2794. Currently, the Phage agent waits until the remote command completes before sending output back to the controller.

Instead, stream output continuously.

Also work around some kind of weirdness with error reporting that I hit but couldn't immediately figure out (maybe PHP7-related?).

Test Plan: Ran bin/phage remote --hosts ... upgrade, etc., and saw a steady stream of output across multiple hosts.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2794

Differential Revision: https://secure.phabricator.com/D17378