Consider using pm2 instead of systemd

[mattpen]

The pm2 process manager is now in use on phet-server2 (and phet-server-dev) by phet-admin. I don't know if it will be helpful for rosetta, but I wanted to make sure the rosetta devs were aware.

I started using pm2 for the new user services to take advantage of the cluster mode, it is now running as a stateless process on N processors instead of a single threaded process, and it was really easy to set that up. I’m not sure if rosetta will ever have enough demand to make that desirable, but there is a good discussion of the differences between the prominent node based process managers and systemd here: https://expressjs.com/en/advanced/pm.html. Definitely not something to be done in a hurry, just something to consider for rosetta 2.1 or 3.0 perhaps.