pay does this, xpay does not. Which means if a block comes in (or you're behind),
you get gratuitous failures:
```
def test_xpay_simple(node_factory):
l1, l2, l3, l4 = node_factory.get_nodes(4, opts={'may_reconnect': True})
node_factory.join_nodes([l1, l2, l3], wait_for_announce=True)
node_factory.join_nodes([l3, l4], announce_channels=False)
# BOLT 11, direct peer
b11 = l2.rpc.invoice('10000msat', 'test_xpay_simple', 'test_xpay_simple bolt11')['bolt11']
> ret = l1.rpc.xpay(b11)
tests/test_xpay.py:148:
...
if not isinstance(resp, dict):
raise TypeError("Malformed response, response is not a dictionary %s." % resp)
elif "error" in resp:
> raise RpcError(method, payload, resp['error'])
E pyln.client.lightning.RpcError: RPC call failed: method: xpay, payload: ('lnbcrt100n1pn5qu7csp53rp0mfwtfsyyy8gzsggepnxgslyalwvz3jkg9ptmqq452ln2nmgqpp58ak9nmfz9l93r0fpm266ewyjrhurhatrs05nda0r03p82cykp0vsdp9w3jhxazl0pcxz72lwd5k6urvv5sxymmvwscnzxqyjw5qcqp99qxpqysgqa798258yppu2tlfj8herr3zuz0zgux79zvtx6z57cmfzs2wdesmr4nvnkcmyssyu6k64ud54eg0v45c3mcw342jj6uy7tu202p6klrcp6ljc9w',), error: {'code': 203, 'message': "Destination said it doesn't know invoice: incorrect_or_unknown_payment_details"}
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-None: xpay is new this release.
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).