Add delay limit to `pay`

[ZmnSCPxj]

In LN, there is a delay for each hop, increasing for each forwarding hop. However, the delay for this affects the payer when a node on the route forwards the HTLC and then dies or disconnects before it can receive the response from the next node: the money is effectively locked for this time.

Inspired by #1065.

I would like to do this after #1012 gets merged in.