Message handling optimization

[jannikluhn]

• keypers validate decryption keys even if they have already done so (ie key is in db)
• keypers validate decryption key shares even if they have already done so (ie key is in db)
• keypers aggregate key shares even if they know that they don't have enough yet or have already generated the key
• keypers resend decryption keys when new shares arrive

[jannikluhn]

We do an unnecessary epoch secret key share verification during decryption key share handling: DecryptionKeyShareHandler.HandleMessage -> DecryptionKeyShareHandler.aggregateDecryptionKeySharesFromDB -> epochKG.HandleEpochSecretKeyShare -> VerifyEpochSecretKeyShare

Preliminary benchmarks suggest that this is the bottleneck of the handler. We should check if this can be removed (ie is only called on trusted inputs) and if so remove it. If not, maybe there's a way to pass a flag to disable the check.