When updating the Buildbot configuration, we need to wait for Buildbot to not be executing any jobs before we can safely restart it.

See discussion in #300.

Apparently there is a way to just reload the Buildbot configuration instead of restarting it via SIGHUP or buildbot reconfig, but it's fragile, so I'd prefer not to do that: http://docs.buildbot.net/current/manual/cfg-intro.html?highlight=reconfig#reloading-the-config-file-reconfig

Just to be clear, this is all for the Buildbot master config + service, not the builder machines, yes?