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.

graph/setup_test.go

@@ -0,0 +1,11 @@
+package graph
+
+import (
+	"testing"
+
+	"github.com/lightningnetwork/lnd/kvdb"
+)
+
+func TestMain(m *testing.M) {
+	kvdb.RunTests(m)
+}