feature(worker): limit rsync memory using cgroup

systemd/tunasync-worker.service

@@ -6,10 +6,10 @@ After=network.target
 Type=simple
 User=tunasync
 PermissionsStartOnly=true
-ExecStartPre=/usr/bin/cgcreate -t tunasync -a tunasync -g cpu:tunasync
+ExecStartPre=/usr/bin/cgcreate -t tunasync -a tunasync -g memory:tunasync
 ExecStart=/home/bin/tunasync worker -c /etc/tunasync/worker.conf --with-systemd
 ExecReload=/bin/kill -SIGHUP $MAINPID
-ExecStopPost=/usr/bin/cgdelete cpu:tunasync
+ExecStopPost=/usr/bin/cgdelete memory:tunasync
 
 [Install]
 WantedBy=multi-user.target