The spec says that the required signatures for a signed event is the
signature of sender's server (unless is a third party invite) and the
`event_id` server (in v1 and v2 room versions).
This changes the previous behaviour, which tried to verify the
signatures for all the servers in the `PublicKeyMap`, instead of
checking only the required signatures.
Relevant spec section:
https://matrix.org/docs/spec/server_server/r0.1.4#validating-hashes-and-signatures-on-received-events
