Make sure we've actually confirmed the HTLC; if it's not confirmed yet
then we won't fast-fail it, and we'll timeout instead:
```
> l1.rpc.waitsendpay(payment_hash=inv['payment_hash'], timeout=TIMEOUT, partid=1)
E AssertionError: Pattern 'WIRE_PERMANENT_CHANNEL_FAILURE \\(reply from remote\\)' not found in "RPC call failed: method: waitsendpay, payload: {'payment_hash': 'c186643391469aa8190415496c85b1eb789cb2b756a76d4c9ce21dd34c698d92', 'timeout': 30, 'partid': 1}, error: {'code': 200, 'message': 'Timed out while waiting'}"
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>