Switch NeonVMs to systemd

[sharnoff]

Problem description / Motivation

There's a few reasons why we might want to switch to using systemd:

1. Better lifecycle management for "daemons", which may help with #577
2. Ability to use preexisting logging infrastructure like journalctl, for #578
3. It's a more "standard" init system, so should hopefully make NeonVM easier to hack on and maintain

DoD

NeonVMs use systemd as the init system.

Implementation ideas

This probably also involves switching where we get all the binaries and shared libs — currently those are from alpine, but switching to debian would probably make this much easier. Relevant code is here:

https://github.com/neondatabase/autoscaling/blob/8a91e83210452e5421e81e755d8f97f363d03e16/neonvm/tools/vm-builder/files/Dockerfile.img#L19-L51

(and, maybe this could be easier by just chroot + install? or, it'll be much easier if we're running proper containers inside the VM, because then we don't need to worry about isolating host files)