multi: add keep-failed-payment-attempts flag

sample-lnd.conf

@@ -311,6 +311,10 @@
 ; a new commitment.
 ; channel-commit-batch-size=10
 
+; Keeps persistent record of all failed payment attempts for successfully
+; settled payments.
+; keep-failed-payment-attempts=false
+
 ; The default max_htlc applied when opening or accepting channels. This value
 ; limits the number of concurrent HTLCs that the remote party can add to the
 ; commitment. The maximum possible value is 483.