Synchronize all payment attempt results

[JssDWt]

Swaps are locked for payment when doing a payout. If the server restarts, or for whatever reason the lock is not removed after the payment, the swap can be locked for payment forever. swapd already synchronizes preimages in the background, but not yet failed payments. swapd should synchronize payments in the background to ensure locks are removed when the payment is no longer in progress.

Suggestion to check whether a payment is in-progress:

• payment is in a 'pending' state
• AND there are no htlcs in-flight
• AND the last started payment attempt was a reasonable time ago, perhaps 30 minutes

This is a leftover from #28