Fix write replay ordering when immediate_commit != all

Previous implementation didn't respect write ordering and could lead
to corrupted data when restarting writes after an OSD outage

Also rework cluster_client queueing logic and add tests for it to verify the correct behaviour

src/mock/ringloop.h

@@ -0,0 +1,25 @@
+// Copyright (c) Vitaliy Filippov, 2019+
+// License: VNPL-1.1 or GNU GPL-2.0+ (see README.md for details)
+
+#pragma once
+
+#include <functional>
+
+struct ring_consumer_t
+{
+    std::function<void(void)> loop;
+};
+
+class ring_loop_t
+{
+public:
+    void register_consumer(ring_consumer_t *consumer)
+    {
+    }
+    void unregister_consumer(ring_consumer_t *consumer)
+    {
+    }
+    void submit()
+    {
+    }
+};