[mem] Fix nil pointer error in previous commit
[mem] Fix nil pointer error in previous commit

- forgot to use NewWriteServiceImpl in `server.go`

xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k,xephonhq/xephon-k