loki: shutdown module before shutting down HTTP server

Shutting the module down before shutting down the HTTP server enables
metrics to continue to be collected while the module terminates.

This change syncs up with how Cortex modules are shut down; the module
is always terminated before the HTTP server.

Fixes grafana#819.

pkg/loki/loki.go

@@ -139,8 +139,8 @@ func (t *Loki) Run() error {
 // Stop gracefully stops a Loki.
 func (t *Loki) Stop() error {
 	t.stopping(t.cfg.Target)
-	t.server.Shutdown()
 	t.stop(t.cfg.Target)
+	t.server.Shutdown()
 	return nil
 }