v0.9.2 an event at connect time is shown once, not twice

Fixed. An event emitted after a page subscribed to /chat/events but before its response body started was shown twice: once as replay, once as live. The history snapshot is now taken at the moment of subscribing, so the boundary between replay and live is that instant and nothing lands on both sides of it. Contributed by @Iams4kura in #1229, with a test that fails on the previous code.