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.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/16584.misc b/changelog.d/16584.misc
new file mode 100644
index 0000000000..beec8f2301
--- /dev/null
+++ b/changelog.d/16584.misc
@@ -0,0 +1 @@
+Simplify persistance code to be per-room.