Sliding Sync: Fix _bulk_get_max_event_pos(...) being inefficient

[MadLittleMods]

Fix _bulk_get_max_event_pos(...) being inefficient. It kept adding all of the batch_results to the results over and over every time we checked a single room in the batch.

I think we still ended up with the right answer before because we accumulate recheck_rooms and actually recheck them to overwrite the bad data we wrote to the results before.

Introduced in https://github.com/element-hq/synapse/pull/17606/files#diff-cbd54e4b5a2a1646299d659a2d5884d6cb14e608efd2e1658e72b465bb66e31bR1481

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