Return error when trying to incentivize an already relayed packet

[crodriguezvega]

To prevent that a packet that has already been relayed can still be incentivized (and therefore fees end up being locked up forever), on PayPacketFeeAsync, if the NextSequenceSend is greater than the packet sequence that is intended to be incentivized AND the packet commitment does not exist (since packet commitments are deleted when packets are acknowledged or timed out), return an error.

[colin-axner]

if packet is not sent {
    return err
}

if packet acknowledged || timed out {
    return err
}