Discard stdout/stderr from the aphlict subprocess when running in daemon (normal) mode
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.
Reviewed By: chad
Maniphest Tasks: T11818
Differential Revision: https://secure.phabricator.com/D16855