Directly start Synapse processes instead of using supervisor for Docker image with workers

[clokep]

This attempts to fix #10065.

This takes the suggestion in there to manually start the Synapse processes and wait for it to be ready via the SDNOTIFY stuff (instead of depending on supervisor to start the processes).

Unfortunately at the end of this (while using complement) I seem to end up at the same result as when using supervisor. Complement intermittently cannot get a /versions response.

I'm also unsure if this makes sense if we're asking people to use this image outside of complement since this has no process monitoring.

The core of the change is in the second commit, but I'm not really sure where to go from here. 😢

I was testing this with: COMPLEMENT_DIR=<...> WORKERS=1 ./scripts-dev/complement.sh TestVersionStructure from the synapse root

[clokep]

I'm going to close this, but if someone wants to pick this open do feel free to reuse this branch / PR!