htlcswitch+routing: implement expiry_too_far failure

In this commit we add a check to HtlcSatifiesPolicy to verify that the
time lock for the outgoing htlc that is requested in the onion packet
isn't too far in the future.

Without this check, anyone could force an unreasonably long time lock on
the forwarding node.

routing/router.go

@@ -1957,6 +1957,21 @@ func (r *ChannelRouter) sendPayment(payment *LightningPayment,
 				)
 				continue
 
+			// If we crafted a route that contains a too long time
+			// lock for an intermediate node, we'll prune the node.
+			// As there currently is no way of knowing that node's
+			// maximum acceptable cltv, we cannot take this
+			// constraint into account during routing.
+			//
+			// TODO(joostjager): Record the rejected cltv and use
+			// that as a hint during future path finding through
+			// that node.
+			case *lnwire.FailExpiryTooFar:
+				pruneVertexFailure(
+					paySession, route, errSource, false,
+				)
+				continue
+
 			// If we get a permanent channel or node failure, then
 			// we'll note this (exclude the vertex/edge), and
 			// continue with the rest of the routes.