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/messenger.cpp

@@ -55,3 +55,10 @@ json11::Json::object osd_messenger_t::merge_configs(const json11::Json::object &
 {
     return cli_config;
 }
+
+bool json_is_true(const json11::Json & val)
+{
+    if (val.is_string())
+        return val == "true" || val == "yes" || val == "1";
+    return val.bool_value();
+}