Validate decryption keys

[jannikluhn]

when the decryptor receives a key, they should check that it's valid. They can do so by comparing it against the eon public key and the epoch id. The eon public key is hardcoded as a constant somewhere -- this is a temporary solution. In the future, we might read it from Shuttermint or from a different blockchain

[jannikluhn]

Since #57 there's a check that the key decodes and a function was added to check that the key is correct. We cannnot call it yet though as we don't know or store the eon key yet.

[jannikluhn]

In #51 we decided we'll go for a key broadcast contract. Since we don't have one yet, for now we'll just assume the key exists in the db.