Enable trace discard mode for overseers

Summary: We enable this for daemons but not for overseers. Overseers should also discard events; this prevents them from buffering a log of indefinite size.

Test Plan:
Ran:

./scripts/daemon/launch_daemon.php --trace --verbose --trace-memory PhutilNiceDaemon --conduit-uri=http://local.aphront.com/api/

Observed steady memory usage.

Reviewers: nh, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1636

Differential Revision: https://secure.phabricator.com/D3238