Workaround for hang in dockerd when reading a tarfile over http

connection. Is this the only place where the race can occur in dockerd?
A workaround fix in dockerd, per

   https://groups.google.com/forum/#!msg/golang-nuts/PMm8nH0yaoA/sWDpPpjcdk8J

suggests that putting a read timeout in the http connection in
dockerd will mask the race condition.

I don't have time to hack on dockerd right now so I propose the following
temporary fix.