Send daemon heartbeat, exit status

Summary:
Send a heartbeat via conduit (if possible) for daemons, and when they exit
cleanly, set the status to exited. This is the libphutil part of D3126.

Test Plan: See test plan for D3126.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3127