The remove hook implementation was copy/pasted from the line above and
I didn't change the body, resulting in packet forwarding routes never
being removed.
Fortunately we weren't using this path yet, but it led to stats being
off, and (very) slow memory growth.
The magicsock derpReader was holding onto 65KB for each DERP
connection forever, just in case.
Make the derp{,http}.Client be in charge of memory instead. It can
reuse its bufio.Reader buffer space.
