Revert "Merge pull request #283 from matrix-org/erikj/atomic_join_federation"
Revert "Merge pull request #283 from matrix-org/erikj/atomic_join_federation"

This reverts commit 5879edbb097d19c2b5f5e064841909e67d6018fe, reversing
changes made to b43930d4c9479973fb7225818e2e51bec022c4e6.

matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,iot-factory/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,iot-factory/synapse,iot-factory/synapse,matrix-org/synapse,iot-factory/synapse,matrix-org/synapse,TribeMedia/synapse,iot-factory/synapse,TribeMedia/synapse,TribeMedia/synapse