Simplify persistance code

[clokep]

We only ever persist events for a single room at once, but the code handles multiple rooms at once this:

• Makes the code harder to read.
• Creates an objects which map room ID -> whatever unnecessarily.

I'm not sure if this is desirable if we ever want to be able to persist from multiple rooms at once, but... 🤷

The first commit factors out a method, so should be reviewed separately.