R1:9309723ac477

Send graceful shutdown signals to daemons in Phabricator

Summary:
Fixes T5855. Adds a `--graceful N` flag to `phd stop` and `phd restart`.

`phd` will send SIGINT, wait `N` seconds, SIGTERM, wait 15 seconds, and SIGKILL. By default, `N` is 15.

Test Plan:
- Ran `bin/phd debug ...` and used `^C` to interrupt daemons. Saw graceful shutdown behavior, and abrupt termination on multiple `^C`.
- Ran `bin/phd start`, `bin/phd stop` and `bin/phd restart` with `--graceful` set to various things, notably `0`.