multi: remove the need for the graphsession package

In this commit, we add a `GraphSession` method to the `ChannelGraph`.
This method provides a caller with access to a `NodeTraverser`. This is
used by pathfinding to create a graph "session" overwhich to perform a
set of queries for a pathfinding attempt. With this refactor, we hide
details such as DB transaction creation and transaction commits from the
caller. So with this, pathfinding does not need to remember to "close
the graph session". With this commit, the `graphsession` package may be
completely removed.

routing/integrated_routing_test.go

@@ -404,5 +404,5 @@ func TestPaymentAddrOnlyNoSplit(t *testing.T) {
 	// The payment should have failed since we need to split in order to
 	// route a payment to the destination, but they don't actually support
 	// MPP.
-	require.Equal(t, err.Error(), errNoPathFound.Error())
+	require.ErrorIs(t, err, errNoPathFound)
 }