[BUG] Skyway nonce is not reset

[verabehr]

I've done some digging and my observations are consistent with all agreed upon nonces per chain being reset to 0, while the individual reports of validators have not been reset. I think this line here is the culprit: https://github.com/palomachain/paloma/blob/master/x/skyway/keeper/keeper.go#L113 as it modifies the underlying transaction during iteration, which smells like the iterator would become invalid or unpredictable.

Instead of deleting the key, we can just reset it to 0 as well I think, OR we could delete all keys on record for this store. Either should work.