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.

netann/channel_update_test.go

@@ -7,11 +7,11 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
+	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/netann"
-	"github.com/lightningnetwork/lnd/routing"
 )
 
 type mockSigner struct {
@@ -182,7 +182,7 @@ func TestUpdateDisableFlag(t *testing.T) {
 
 			// Finally, validate the signature using the router's
 			// verification logic.
-			err = routing.VerifyChannelUpdateSignature(
+			err = graph.VerifyChannelUpdateSignature(
 				newUpdate, pubKey,
 			)
 			if err != nil {