Check that signatures on events are valid
Check that signatures on events are valid

We should check that both the sender's server, and the server which created the
event_id (which may be different from whatever the remote server has told us
the origin is), have signed the event.

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