So far we've always been deferring the deletion, retry and early abort
logic to `sendonion` and `sendpay` which do not have the context to
decide if a call is legitimate or not (they were mostly based on
heuristics). By calling `listsendpays` for the invoice's
`payment_hash` we can identify what our `groupid` should be, but more
importantly we can also abort if another payment is pending or a prior
attempt has already succeeded.
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).