Allow specifying retry strategy per-payment instead of globally

[TheBlueMatt]

Probably should have thought of this before #1418, but oh well - ideally we'd be able to specify the payment retry count on a per-payment basis rather than globally in the InvoicePayer. This would likely imply refusing to retry things that weren't started using the InvoicePayer/after restart, but that may be okay? Would have to document that or make it configurable.

[benthecarman]

Tried working on this, seems complications will come from handle_event_internal. Could simplify by keeping the current retry strategy as the default one and have the ability to pass an Option<Retry> when paying to override

[valentinewallace]

Ah, this is being done as a side effect of #1916 and its follow-ups actually. Guess I'll self-assign.

[simonadameit]

It appears this issue is resolved. Could it be closed, or is there any further clarification needed?

[valentinewallace]

It appears this issue is resolved. Could it be closed, or is there any further clarification needed?

Good point, yes this should be resolved.