check pending invoices only once on startup,
then check outgoing payments every 30 minutes,
and delete outgoing payments that return False (meaning they have failed).
also fix a bug on sparko.
before we were using requests which had no default timeouts, but httpx has a
default timeout of 5 seconds. should have noticed that earlier.
when the timeout expires we are left with a pending payment on the db with a
temporary checking_id so we can never know if it was completed or not.
this is still an issue, because technically a lightning payment may take 2 weeks
or more, and we must have a way to dispatch a payment and check for it later.
that should be the default (and we already do check for the payment status later,
so half of the work is done), but on the other hand backends like lnpay and
opennode do not give us a checking_id before the thing is already settled.
