The key assignment feature can't be used to verify equivocation

[sainoe]

Problem

The key assignment feature uses the chainID | providerConsAddress -> consumerKey mapping to store consumer public keys. It entails that only the last assigned consumer public key is stored for each validator and the old ones are overwritten.

In the scenario where a validator double votes on a consumer chain using key k1, it potentially has enough time to re-assign itself a new key, k2, on the consumer chain before the double voting evidence is detected and submitted to the provider. Consequently, when the evidence is eventually submitted, it will be verified using k2 and fail.

This issue supersedes #1254, which partially fixed the validator public key retrieval in the double voting evidence handler.

Closing criteria

The public key of the validator that double voted should be directly retrieved from the infraction_block_header field in the SubmitConsumerDoubleVoting message, rather than from the provider states.

Credits to @insumity for reporting the bug 💜.