Don't use a separate LinkService for DAGService.GetLinks()

Instead make LinkService a part of DAGService.  The LinkService is now
simply an interface that DAGService implements.  Also provide a
GetOfflineLinkService() method that the GC uses to get an offline
instance.

License: MIT
Signed-off-by: Kevin Atkinson <k@kevina.org>

offline.go

@@ -67,3 +67,7 @@ func (e *offlineExchange) GetBlocks(ctx context.Context, ks []key.Key) (<-chan b
 	}()
 	return out, nil
 }
+
+func (e *offlineExchange) IsOnline() bool {
+	return false
+}