refactor: event handling code

src/api/client_server/membership.rs

src/api/server_server.rs

src/service/rooms/alias/mod.rs

src/service/rooms/directory/mod.rs

src/service/rooms/edus/mod.rs

src/service/rooms/event_handler/mod.rs

src/service/rooms/lazy_loading/mod.rs

src/service/rooms/metadata/mod.rs

src/service/rooms/outlier/mod.rs

src/service/rooms/pdu_metadata/mod.rs

src/service/rooms/search/mod.rs

src/service/rooms/short/mod.rs

src/service/rooms/state/data.rs

src/service/rooms/state/mod.rs

src/service/rooms/state_accessor/mod.rs

src/service/rooms/state_cache/mod.rs

src/service/rooms/state_compressor/mod.rs

src/service/rooms/timeline/mod.rs

src/service/rooms/user/mod.rs