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.go

@@ -1608,7 +1608,13 @@ func (l *channelLink) handleUpstreamMsg(msg lnwire.Message) {
 			return
 		}
 
-		// TODO(roasbeef): pipeline to switch
+		settlePacket := &htlcPacket{
+			outgoingChanID: l.ShortChanID(),
+			outgoingHTLCID: idx,
+			htlc: &lnwire.UpdateFulfillHTLC{
+				PaymentPreimage: pre,
+			},
+		}
 
 		// Add the newly discovered preimage to our growing list of
 		// uncommitted preimage. These will be written to the witness
@@ -1616,6 +1622,9 @@ func (l *channelLink) handleUpstreamMsg(msg lnwire.Message) {
 		// from the remote peer.
 		l.uncommittedPreimages = append(l.uncommittedPreimages, pre)
 
+		// Pipeline this settle, send it to the switch.
+		go l.forwardBatch(settlePacket)
+
 	case *lnwire.UpdateFailMalformedHTLC:
 		// Convert the failure type encoded within the HTLC fail
 		// message to the proper generic lnwire error code.