Implement client writeback cache

- Disabled by default, enable with client_enable_writeback=true
- Even then only enabled in FIO when -direct is disabled and in QEMU when
  block device cache is enabled in settings
- Can also be enabled in other clients like vitastor-cli using parameter
  client_writeback_allowed=true, but not recommended

src/mock/ringloop.h

@@ -22,4 +22,10 @@ public:
     void submit()
     {
     }
+    void wait()
+    {
+    }
+    void loop()
+    {
+    }
 };