drain: fix double-close panic on drain future
drain: fix double-close panic on drain future

nak3/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,nak3/nomad,burdandrei/nomad,nak3/nomad,burdandrei/nomad,hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,nak3/nomad,dvusboy/nomad,dvusboy/nomad,nak3/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,nak3/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,nak3/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad