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/nbd_proxy.cpp

@@ -341,6 +341,7 @@ public:
             ringloop->loop();
             ringloop->wait();
         }
+        cli->flush();
         delete cli;
         delete epmgr;
         delete ringloop;