Disallow invoices below allowed_underpay_amount

[kristapsk]

Such invoices are otherwise autoconfirmed, without user making a payment at all. Fixes #92.

End user will have the same "Amount below dust limit" message, as I don't think they should know details about configuration. But what came into my mind - should we maybe also output this amount, which would be allowed_underpay_amount for LN, max(allowed_underpay_amount,onchain_dust_limit) for onchain?

Also, likely these log messages should not be warnings, but info, as it's user generated action, not something wrong on server side.

[nickfarrow]

Agree, I don't have any preference as to whether users see the value. But the value isn't exactly private since you could binary search and find it quickly. Yep that max is good if we do want to display the value.

It's a shame 1 sat does not work. I'll see if i can try fix the amounts rounding

[kristapsk]

But the value isn't exactly private since you could binary search and find it quickly.

Don't think it even needs to be private. For example, in case of donation page, it's good for user to know minimum amount he can pay.

[kristapsk]

I'll see if i can try fix the amounts rounding

When rounding issue is solved, allowed_underpay_amount can be changed to work for onchain payments only as with LN invoices exact amount will be always paid.

[kristapsk]

I'll see if i can try fix the amounts rounding

#164 should do it.