Implement TTL for receivables scan, to ensure if there is no response that the next scan can occur

[kauri-hero]

This card is currently blocked by #676 and #744. Read more down here in the comment.

This may be a continuation of card #611

'Scan already in progress' Error appears for receivables scans of the blockchain - overall it appears that if there was no response from a receivables scan, there is no time-out.

Need to implement a TTL for receivables scan, and if there is a timeout, then the next scan can be attempted.

"The code isn't observing any blockchain service timeout"

[dnwiebe]

DANGER: Make sure you handle the situation where you send Request 1 and time out and give up, then send Request 2, then receive Response 1 (hey--it wasn't dead after all!), then receive Response 2. If you process both Response 1 and Response 2, you'll record duplicate payments. You have to reject one of them.

Dan has some ideas about this. If you can talk to him, do so; if you can't, think about keeping track of what the start block was when you send each request, and using it to tell whether an incoming response is unique or overlapping.

[bertllll]

If we get started to haunt the objectives of this card now, we might bring more difficulties into our already tough position. See the newly created card #744 in case you're seeking for more details.