Clearing cache doesn't stop client first

[dbkr]

The 'clear cache & reload' button doesn't stop the client before it nukes all the data, which is normally fine because it reloads the app immediately after, but if for any reason it doesn't (in my case, because of #3597), it will wipe all the data from the store, then next time something comes down the sync stream, get very confused and start emitting spurious new room events, etc., which can also caused data loss because it decided I had no m.direct event and so re-guessed my DM rooms, found none, then replaced my m.direct event with the empty object.

[dbkr]

This was causing the data loss above in real situations since the indexeddb store waits on a promise while the database is deleted which gives adequate time for a sync response to return.