connpool: fix accidental removal of atomics use for conn.closed.
connpool: fix accidental removal of atomics use for conn.closed.

alphagov/radix,gabrielperezs/radix.improved,mediocregopher/radix.v2,trigrass2/radix,rightscale/radix,ctrlaltdel121/radix,mediocregopher/radix.v3,beni55/radix,bpowers/radix,fzzy/radix,bpowers/radix