refactor: move various duties from ChannelRouter to graph.Builder

This commit is a large refactor that moves over various responsibilities
from the ChannelRouter to the graph.Builder. These include all graph
related tasks such as:
- graph pruning
- validation of new network updates & persisting new updates
- notifying topology update clients of any changes.

This is a large commit but:
- many of the files are purely moved from `routing` to `graph`
- the business logic put in the graph Builder is copied exactly as is
  from the ChannelRouter with one exception:
- The ChannelRouter just needs to be able to call the Builder's
  `ApplyChannelUpdate` method. So this is now exported and provided to
the ChannelRouter as a config option.
- The trickiest part was just moving over the test code since quite a
  bit had to be duplicated.

discovery/chan_series.go

@@ -5,9 +5,9 @@ import (
 
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/netann"
-	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
 
@@ -136,7 +136,7 @@ func (c *ChanSeries) UpdatesInHorizon(chain chainhash.Hash,
 		if edge1 != nil {
 			// We don't want to send channel updates that don't
 			// conform to the spec (anymore).
-			err := routing.ValidateChannelUpdateFields(0, edge1)
+			err := graph.ValidateChannelUpdateFields(0, edge1)
 			if err != nil {
 				log.Errorf("not sending invalid channel "+
 					"update %v: %v", edge1, err)
@@ -145,7 +145,7 @@ func (c *ChanSeries) UpdatesInHorizon(chain chainhash.Hash,
 			}
 		}
 		if edge2 != nil {
-			err := routing.ValidateChannelUpdateFields(0, edge2)
+			err := graph.ValidateChannelUpdateFields(0, edge2)
 			if err != nil {
 				log.Errorf("not sending invalid channel "+
 					"update %v: %v", edge2, err)