multi: don't access loop variables in goroutines

This commit makes sure that no loop variables or other temporary
variables are accessed directly in a goroutine but are instead passed
into the goroutine through a parameter. This makes sure a copy of the
value is put on the stack and is not changed while the outside loop
continues.

lntest/itest/utils.go

@@ -78,13 +78,12 @@ func completePaymentRequests(client lnrpc.LightningClient,
 	// Launch all payments simultaneously.
 	results := make(chan error)
 	for _, payReq := range paymentRequests {
-		payReqCopy := payReq
-		go func() {
-			err := send(payReqCopy)
+		go func(payReq string) {
+			err := send(payReq)
 			if awaitResponse {
 				results <- err
 			}
-		}()
+		}(payReq)
 	}
 
 	// If awaiting a response, verify that all payments succeeded.