Discard stdout/stderr from the aphlict subprocess when running in daemon (normal) mode

Summary:
Fixes T11818. We don't discard output, so once we read more than 2GB of output we'll exceed the maximum size of a string in an internal buffer.

Instead, configure the future so output is discarded.

Test Plan: Added logging to libphutil/, saw internal buffer grow steadily before this change and stay constant at 0 after this change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11818

Differential Revision: https://secure.phabricator.com/D16855