Use counterparty upgrade version in upgrade ack if current channel state is OPEN

[damiannolan]

Summary

Add the following logic to ChanUpgradeAck:

If current channel state is OPEN then we accept the counterparty upgrade version as the final upgrade version by assigning it to self upgrade version field.

// optimistically accept version that TRY chain proposes and pass this to callback for confirmation
// in the crossing hello case, we do not modify version that our TRY call returned and instead enforce
// that both TRY calls returned the same version
if currentChannel.state == OPEN {
    upgrade.fields.version == counterpartyUpgrade.fields.version
}

spec ref

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