routing+discovery: extract local channel manager

The policy update logic that resided part in the gossiper and
part in the rpc server is extracted into its own object.

This prepares for additional validation logic to be added for policy
updates that would otherwise make the gossiper heavier.

It is also a small first step towards separation of our own channel data
from the rest of the graph.

discovery/gossiper_test.go

@@ -3602,20 +3602,26 @@ out:
 	// Now that all of our channels are loaded, we'll attempt to update the
 	// policy of all of them.
 	const newTimeLockDelta = 100
-	newPolicy := routing.ChannelPolicy{
-		TimeLockDelta: newTimeLockDelta,
-	}
-	newChanPolicies, err := ctx.gossiper.PropagateChanPolicyUpdate(newPolicy)
+	var edgesToUpdate []EdgeWithInfo
+	err = ctx.router.ForAllOutgoingChannels(func(
+		info *channeldb.ChannelEdgeInfo,
+		edge *channeldb.ChannelEdgePolicy) error {
+
+		edge.TimeLockDelta = uint16(newTimeLockDelta)
+		edgesToUpdate = append(edgesToUpdate, EdgeWithInfo{
+			Info: info,
+			Edge: edge,
+		})
+
+		return nil
+	})
 	if err != nil {
-		t.Fatalf("unable to chan policies: %v", err)
+		t.Fatal(err)
 	}
 
-	// Ensure that the updated channel policies are as expected.
-	for _, dbPolicy := range newChanPolicies {
-		if dbPolicy.TimeLockDelta != uint16(newPolicy.TimeLockDelta) {
-			t.Fatalf("wrong delta: expected %v, got %v",
-				newPolicy.TimeLockDelta, dbPolicy.TimeLockDelta)
-		}
+	err = ctx.gossiper.PropagateChanPolicyUpdate(edgesToUpdate)
+	if err != nil {
+		t.Fatalf("unable to chan policies: %v", err)
 	}
 
 	// Two channel updates should now be broadcast, with neither of them