Recover from failed epoch key generation attempts

[jannikluhn]

If too few keypers are online, epoch key generation will naturally fail. When new keypers come online though, we should recover from this. At the moment though, we don't since the newly joined keypers will have missed the decryption trigger message.

A simple way to fix this is to make the collator (or the keypers?) resend the decryption trigger in regular intervals until the key has been generated.

Keypers should reject triggers if they recently saw it or the key has been successfully generated already in order to prevent simple spam attacks.

[jannikluhn]

probably needs #220

[jannikluhn]

outdated