sometimes dummy_event sending can't keep up with new extremities

This issue has been migrated from [#7682](https://github.com/matrix-org/synapse/issues/7682).

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For high-volume rooms, it's possible for forward extremities to accumulate faster than the dummy_event sender clears them out.

once a certain number of forward extremities have accumulated, it takes a long time for each new event to be processed, increasing the chance of several arriving in parallel, thus increasing the extremity count further. (Likewise, it takes a long time to send each dummy event, limiting the rate at which they can be sent.)