MSC2716 federation changes (#175)

 - Update and uncomment MSC2716 federation test cases
    - matrix-org/synapse#10498
    - matrix-org/synapse#10245
 - Simplify `batchSendHistoricalMessages` function signature by using sorta composable functions
 - More proper waiting to avoid flakey test
 - Make error comment match helper function name
 - Add test for private room backfilling
 - Verify we only return state events that we pass in, matrix-org/synapse#10552

Author: MadLittleMods