Add receiver-side enforced amount constraint for zero-valued invoices

[joostjager]

When a zero-valued invoice is created, the receiver has no control over the amount that is possibly accepted. This can be fixed by creating the invoice as a hodl invoice and inspecting the amount before settling. An external application is required though for tracking the invoices.

With recurring AMP payments, it arguably is more important to have control over the amount range and in particular the minimum amount. Otherwise someone can fill up the invoice db with large numbers of recurring 1 msat payments. The fix above cannot be used, because AMP-hodl invoices are not (yet) supported.

An alternative would be to consider the amount range as a basic property of a zero-valued amp or regular invoice and enforce it in the invoice registry.

This change would also make recurring AMP almost completely compatible with lnd-based wallets that support ln address. See https://twitter.com/joostjgr/status/1455503401366827008

[alexbosworth]

Another approach would be allowing for deletion of settled invoices

[joostjager]

Take the money, but delete the record - how is that related to enforcing an amount range?

[alexbosworth]

Otherwise someone can fill up the invoice db with large numbers of recurring 1 msat payments.

Allowing you to manage your invoice db allows you to mitigate this issue and still receive 1 msat payments