routing: account for case where final destination send TemporaryChannelFailure

In this commit, we fix an existing bug that could cause lnd to crash if
we sent a payment, and the *destination* sent a temp channel failure
error message. When handling such a message, we’ll look in the nextHop
map to see which channel was *after* the node that sent the payment.
However, if the destination sends this error, then there’ll be no entry
in this map.

To address this case, we now add a prevHop map. If we attempt to lookup
a node in the nextHop map, and they don’t have an entry, then we’ll
consult the prevHop map.

We also update the set of tests to ensure that we’re properly setting
both the prevHop map and the nextHop map.

routing/missioncontrol.go

@@ -222,6 +222,10 @@ func (p *paymentSession) RequestRoute(payment *LightningPayment,
 	// shrinking.
 	pruneView := p.pruneViewSnapshot
 
+	log.Debugf("Mission Control session using prune view of %v "+
+		"edges, %v vertexes", len(pruneView.edges),
+		len(pruneView.vertexes))
+
 	// TODO(roasbeef): sync logic amongst dist sys
 
 	// Taking into account this prune view, we'll attempt to locate a path