Fix room membership race with PREPARED event (#2613)
* Fix room membership race with PREPARED event
See the call site of the original triggering event of this function: https://github.com/matrix-org/matrix-js-sdk/blob/b265d795a427c6d30ccdf279a09f7836509df863/src/sliding-sync.ts#L789-L806
I think the bug is current code assumes downstream event listeners of `SlidingSyncEvent.RoomData` have synchronous execution so that by the time it emits `SlidingSyncState.Complete`, and eventually `SyncState.Prepared` the room state is correct. But since SlidingSyncSdk's `processRoomData` is async, and the membership field was being set after the async, it looks like `SlidingSyncState.Complete` was being fired before the membership field was set.
* Rm whitespace
