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.

autopilot/manager.go

@@ -6,9 +6,9 @@ import (
 
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/graph"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
-	"github.com/lightningnetwork/lnd/routing"
 )
 
 // ManagerCfg houses a set of values and methods that is passed to the Manager
@@ -36,7 +36,7 @@ type ManagerCfg struct {
 
 	// SubscribeTopology is used to get a subscription for topology changes
 	// on the network.
-	SubscribeTopology func() (*routing.TopologyClient, error)
+	SubscribeTopology func() (*graph.TopologyClient, error)
 }
 
 // Manager is struct that manages an autopilot agent, making it possible to