Extend send dummy event to remove really old extremities

This issue has been migrated from [#8725](https://github.com/matrix-org/synapse/issues/8725).

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The current send dummy event mechanism will try and reduce the number of extremities in a room to below a threshold. However, if one of the extremities is really old then state res across a small number of extremities can still take quite a lot of resources. We should change the behaviour of the mechanism to try and remove old extremities, even if the count of extremities is below the threshold.

Start by locating Synapse's current send dummy event mechanism and trace how it counts room extremities and applies its threshold. The change is complete when the mechanism also removes sufficiently old extremities even when the total count is below that threshold; the issue names no files or tests.

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