htlcswitch: pipeline settles to switch

This commit makes the outgoing link pipeline the settle to the
switch as soon as it receives it. Previously, it would wait for a
revocation before sending it, which caused increased latency on
payments as well as possibly never settling on the incoming link.
A duplicate settle is still sent to the switch, but it is handled
gracefully. A new AckEventTicker was added to the switch which
acknowledges any pending settle / fail entries in an outgoing
link's fwd pkgs in batch. This was needed in order to reduce the
number of db txn's which would have been incurred by acking whenever
we receive a duplicate settle without batching.

htlcswitch/link_test.go

@@ -232,10 +232,10 @@ func TestChannelLinkSingleHopPayment(t *testing.T) {
 		t.Fatalf("unable to make the payment: %v", err)
 	}
 
-	// Wait for Bob to receive the revocation.
+	// Wait for Alice to receive the revocation.
 	//
 	// TODO(roasbeef); replace with select over returned err chan
-	time.Sleep(100 * time.Millisecond)
+	time.Sleep(2 * time.Second)
 
 	// Check that alice invoice was settled and bandwidth of HTLC
 	// links was changed.
@@ -494,8 +494,8 @@ func testChannelLinkMultiHopPayment(t *testing.T,
 		t.Fatalf("unable to send payment: %v", err)
 	}
 
-	// Wait for Bob to receive the revocation.
-	time.Sleep(100 * time.Millisecond)
+	// Wait for Alice and Bob's second link to receive the revocation.
+	time.Sleep(2 * time.Second)
 
 	// Check that Carol invoice was settled and bandwidth of HTLC
 	// links were changed.
@@ -3977,7 +3977,8 @@ func TestChannelLinkAcceptOverpay(t *testing.T) {
 		t.Fatalf("unable to send payment: %v", err)
 	}
 
-	time.Sleep(100 * time.Millisecond)
+	// Wait for Alice and Bob's second link to receive the revocation.
+	time.Sleep(2 * time.Second)
 
 	// Even though we sent 2x what was asked for, Carol should still have
 	// accepted the payment and marked it as settled.
@@ -5801,7 +5802,12 @@ func TestChannelLinkHoldInvoiceSettle(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	// Wait for Bob to receive the revocation.
+	// Wait for Alice to receive the revocation. This is needed
+	// because the settles are pipelined to the switch and otherwise
+	// the bandwidth won't be updated by the time Alice receives a
+	// response here.
+	time.Sleep(2 * time.Second)
+
 	if ctx.startBandwidthAlice-ctx.amount !=
 		ctx.n.aliceChannelLink.Bandwidth() {