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.

routing/payment_session.go

@@ -9,6 +9,7 @@ import (
 	"github.com/lightningnetwork/lnd/build"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
@@ -412,7 +413,7 @@ func (p *paymentSession) UpdateAdditionalEdge(msg *lnwire.ChannelUpdate,
 	pubKey *btcec.PublicKey, policy *models.CachedEdgePolicy) bool {
 
 	// Validate the message signature.
-	if err := VerifyChannelUpdateSignature(msg, pubKey); err != nil {
+	if err := graph.VerifyChannelUpdateSignature(msg, pubKey); err != nil {
 		log.Errorf(
 			"Unable to validate channel update signature: %v", err,
 		)