Use Puma's Hot Restart for deployments

[dacook]

Hot Restart has benefits over the default stop/start. This will make deployments more seamless:

A hot restart means that no requests will be lost while deploying your new code, since the server socket is kept open between restarts.

https://puma.io/puma4/file.restart.html

• All platforms: clients with an in-flight request are served responses before the connection is closed gracefully. Puma gracefully disconnects any idle HTTP persistent connections before restarting.

https://github.com/puma/puma/blob/master/docs/restart.md

So we should update the deployment playbook, and probably others to use Hot Restarts.

Note that the puma service is already configured to use the faster Phased Restart for the reload action.
But Hot Restart is generally necessary and safer, because it reloads the Puma master process. This means any updates to Ruby, Puma or Puma's dependent gems will successfully reload the new version.

[dacook]

I think we should update the shared puma restart handler to use that method instead.

I'm not sure of the best way, I think either

• we could update the service ExecReload to use hot-restart, then change the handler to use reloaded. Or
• we could update the handler to run the pumactl hot-restart comment directly.

I think the second option is probably better, because phased-restart is closer to reload than hot-restart is.