Simplify event persistence code (#16584)

The event persistence code used to handle multiple rooms
at a time, but was simplified to only ever be called with a
single room at a time (different rooms are now handled in
parallel). The code is still generic to multiple rooms causing
a lot of work that is unnecessary (e.g. unnecessary loops, and
partitioning data by room).

This strips out the ability to handle multiple rooms at once, greatly
simplifying the code.

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