The rewind picker works out change counts at the same time

The rewind picker lets you go back to an earlier point in a conversation. It shows how much changed at each message. For the most recent messages, it used to work these out one after another. It now works them out at the same time and stops waiting once a time limit runs out.

When there is no earlier point available, the picker can now say "Nothing to rewind to." as well as the existing "Nothing to rewind to yet."