warden writes its own pidfile on startup

This ensures that when the DEA comes back up it can immediately talk to
Warden and reload its instances. Previously the pid file was written too
early, so potentially Warden wasn't actually even listening on the
socket, and hadn't reloaded its containers.

[#57361124]
runner: allow max_in_flight to be overridden
use DEA/Warden snapshotting in drain script

Now sends USR1 once, and backs off until all staging tasks complete.
After that, sends USR2 to Warden, and kill -9's the DEA. No evacuation
is performed. Both jobs will come up and repopulate their state via
their respective snapshot loading mechanisms.

If the job is going to be shut down, execute the existing evacuation
strategy, and wait 2 minutes.
warden: don't destroy containers on startup

We're using the snapshot feature again, so this is counterproductive.

Old containers should be destroyed via the grace period if no one's
actually talking to them.