xpay: limit maximal number of payment parts

[michael1011]

In some scenarios, it is desirable to limit the maximal number of parts (eg Phoenix on the fly channel funding: This payment has been split in two many parts by the sender: 31 parts vs max 6 parts allowed for on-the-fly funding.) when paying an invoice

[Lagrang3]

If a payment was split in so many parts it is likely that it was going to fail anyways.
Many parts indicates that the main routes are almost saturated and usually as we get closer
to those saturation points the theoretical probability of success approaches very quickly to 0.

Anyways, this request is interesting. I think we can write some adhoc modification of the payment routes
to accommodate everything into a small set of routes or fail trying.

Thanks for reporting.