Fix ignored read-receipts
Fix ignored read-receipts

Don't ignore read-receipts which arrive in the same EDU as a read-receipt for
an old event.

matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse