Generate PaymentPathSuccessful event for each path

Generate PaymentPathSuccessful event for each path

A single PaymentSent event is generated when a payment is fulfilled.
This is occurs when the preimage is revealed on the first claimed HTLC.
For subsequent HTLCs, the event is not generated.

In order to score channels involved with a successful payments, the
scorer must be notified of each successful path involved in the payment.
Add a PaymentPathSuccessful event for this purpose. Generate it whenever
a part is removed from a pending outbound payment. This avoids duplicate
events when reconnecting to a peer.