tendermint light client: check_{header,misbehaviour}_and_update_state() missing trusted_validator_set hash check

[plafer]

Bug Summary

Problem is here.

We need to add a check hash(header.trusted_validator_set) == trusted_consensus_state.next_validators_hash. This corresponds to ibc-go's checkTrustedHeader() function.

Description

The light client only keeps the hash of validator sets on chain, as the full set would be too costly to store. Therefore, we rely on the relayer to supply the full validator sets for us when updating a client (in this particular case, stored in Header.trusted_validator_set).

We should never blindly trust what a relayer gives us though. In this case, we should make sure that the relayer passed us the right validator by checking that its hash is the same as the hash we have stored on chain at trusted_consensus_state.next_validators_hash.

Today, we are missing that check, so effectively we're blindly trusting the relayer for that field.