Rendering large MELS is very slow

[ara4n]

If you have tens/hundreds of membership changes in a row, MELS takes a surprisingly long time to render when viewing a timeline for the first time. It's as if every new event added to scrollback calls the MELS (or the whole timeline?) to re-render from scratch. Instead, we should only update the MELS atomically for every batch of scrollback loaded via /messages, rather than on a message by message basis.

[aaronraimist]

Related: #11604 #9200 #9192