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/gossiper_test.go

@@ -33,7 +33,6 @@ import (
 	"github.com/lightningnetwork/lnd/lntest/wait"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/netann"
-	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/lightningnetwork/lnd/ticker"
 	"github.com/stretchr/testify/require"
@@ -351,7 +350,7 @@ func (r *mockGraphSource) IsStaleEdgePolicy(chanID lnwire.ShortChannelID,
 		// Since it exists within our zombie index, we'll check that it
 		// respects the router's live edge horizon to determine whether
 		// it is stale or not.
-		return time.Since(timestamp) > routing.DefaultChannelPruneExpiry
+		return time.Since(timestamp) > graph.DefaultChannelPruneExpiry
 	}
 
 	switch {
@@ -2258,7 +2257,7 @@ func TestProcessZombieEdgeNowLive(t *testing.T) {
 
 	// We'll generate a channel update with a timestamp far enough in the
 	// past to consider it a zombie.
-	zombieTimestamp := time.Now().Add(-routing.DefaultChannelPruneExpiry)
+	zombieTimestamp := time.Now().Add(-graph.DefaultChannelPruneExpiry)
 	batch.chanUpdAnn2.Timestamp = uint32(zombieTimestamp.Unix())
 	if err := signUpdate(remoteKeyPriv2, batch.chanUpdAnn2); err != nil {
 		t.Fatalf("unable to sign update with new timestamp: %v", err)