htlcswitch/link: remove overflow queue

This commit removes the overflowQueue from the link. We do so in order
to promote better UX for senders, so that HTLCs are failed faster when
the commitment is full. This gives the sender the opportunity to try
another, more open path, rather than perceive the HTLC as being stuck.

At the same time, we remove the total number of active goroutines in lnd
by a factor of N where N is the number of active channels.

lntest/itest/lnd_test.go

@@ -10344,9 +10344,7 @@ func testNodeSignVerify(net *lntest.NetworkHarness, t *harnessTest) {
 	closeChannelAndAssert(ctxt, t, net, net.Alice, aliceBobCh, false)
 }
 
-// testAsyncPayments tests the performance of the async payments, and also
-// checks that balances of both sides can't be become negative under stress
-// payment strikes.
+// testAsyncPayments tests the performance of the async payments.
 func testAsyncPayments(net *lntest.NetworkHarness, t *harnessTest) {
 	ctxb := context.Background()
 
@@ -10372,18 +10370,16 @@ func testAsyncPayments(net *lntest.NetworkHarness, t *harnessTest) {
 		t.Fatalf("unable to get alice channel info: %v", err)
 	}
 
-	// Calculate the number of invoices. We will deplete the channel
-	// all the way down to the channel reserve.
-	chanReserve := channelCapacity / 100
-	availableBalance := btcutil.Amount(info.LocalBalance) - chanReserve
-	numInvoices := int(availableBalance / paymentAmt)
+	// We'll create a number of invoices equal the max number of HTLCs that
+	// can be carried in one direction. The number on the commitment will
+	// likely be lower, but we can't guarantee that any more HTLCs will
+	// succeed due to the limited path diversity and inability of the router
+	// to retry via another path.
+	numInvoices := int(input.MaxHTLCNumber / 2)
 
 	bobAmt := int64(numInvoices * paymentAmt)
 	aliceAmt := info.LocalBalance - bobAmt
 
-	// Send one more payment in order to cause insufficient capacity error.
-	numInvoices++
-
 	// With the channel open, we'll create invoices for Bob that Alice
 	// will pay to in order to advance the state of the channel.
 	bobPayReqs, _, _, err := createPayReqs(
@@ -10424,28 +10420,13 @@ func testAsyncPayments(net *lntest.NetworkHarness, t *harnessTest) {
 		}
 	}
 
-	// We should receive one insufficient capacity error, because we sent
-	// one more payment than we can actually handle with the current
-	// channel capacity.
-	errorReceived := false
+	// Wait until all the payments have settled.
 	for i := 0; i < numInvoices; i++ {
-		if resp, err := alicePayStream.Recv(); err != nil {
+		if _, err := alicePayStream.Recv(); err != nil {
 			t.Fatalf("payment stream have been closed: %v", err)
-		} else if resp.PaymentError != "" {
-			if errorReceived {
-				t.Fatalf("redundant payment error: %v",
-					resp.PaymentError)
-			}
-
-			errorReceived = true
-			continue
 		}
 	}
 
-	if !errorReceived {
-		t.Fatalf("insufficient capacity error haven't been received")
-	}
-
 	// All payments have been sent, mark the finish time.
 	timeTaken := time.Since(now)
 
@@ -10535,8 +10516,12 @@ func testBidirectionalAsyncPayments(net *lntest.NetworkHarness, t *harnessTest)
 		t.Fatalf("unable to get alice channel info: %v", err)
 	}
 
-	// Calculate the number of invoices.
-	numInvoices := int(info.LocalBalance / paymentAmt)
+	// We'll create a number of invoices equal the max number of HTLCs that
+	// can be carried in one direction. The number on the commitment will
+	// likely be lower, but we can't guarantee that any more HTLCs will
+	// succeed due to the limited path diversity and inability of the router
+	// to retry via another path.
+	numInvoices := int(input.MaxHTLCNumber / 2)
 
 	// Nodes should exchange the same amount of money and because of this
 	// at the end balances should remain the same.