invoices: jit insert key send invoices

This commit adds handling code for the key send custom record. If this
record is present and its hash matches the payment hash, invoice
registry will insert a new invoice into the database "just in time". The
subsequent settle flow is unchanged. The newly inserted invoice is
picked up and settled. Notifications will be broadcast as usual.

channeldb/invoices.go

@@ -219,7 +219,8 @@ type Invoice struct {
 	// or any other message which fits within the size constraints.
 	Memo []byte
 
-	// PaymentRequest is the encoded payment request for this invoice.
+	// PaymentRequest is the encoded payment request for this invoice. For
+	// spontaneous (key send) payments, this field will be empty.
 	PaymentRequest []byte
 
 	// CreationDate is the exact time the invoice was created.