Consider whether to support looping payments

[tnull]

A user encountered a case where they wanted to pay a JIT-channel invoice generated by ourselves to trigger a channel open from an LSP (Voltage Flow 2.0 in this case).

However, LDK Node rejected the payment with an NodeError: DuplicatePayment: A payment with the given hash has already been initiated error.

Generally, this is may be considered a misuse of JIT channels as they are really 'just-in-time' and not meant to order (and self-pay) liquidity upfront.
While looping payments are kind of bogus, we still should consider if we need to support them if users keep running into similar issues.

[tnull]

After #308 we're going to reject circular payments with a dedicated error method.

Going forward, given that this directly conflicts with short-circuiting self-payments (cf. lightningdevkit/rust-lightning#2462), we're likely not going to support looping/circular payments. Closing as not planned.