Show more details for pending payments

I have a payment that appears to be stuck, for more than 24 hours now:

```
lightning-cli listpays lnbc1…
{
"pays": [
{
"bolt11": "lnbc1...",
"destination": "...",
"payment_hash": "...",
"status": "pending",
"created_at": ...,
"amount_msat": "...msat",
"amount_sent_msat": "...msat"
}
]
}

lightning-cli paystatus lnbc1…
{
"pay": []
}
```

Specifically it would be nice to see what the various possible timeout moments are.

Also, do I assume correctly that "pending" means the pre-image has been revealed to at least one hop on the channel, so it's not safe to pay again? It would be useful to clarify that in the `listpays` doc.

Seeing the entire route(s) would be nice too. Then I can dig around in Mempool to see if e.g. any of the intermediate nodes have been offline or seeing forced channel closes.