onion-tunnel/proxy: fix broken InteractiveCopier

• The InteractiveCopier was subtly broken; if the destination partially wrote something, it would duplicate data (in pathological cases, looping forever).
  • Thanks to @trinity-1686a for catching the bug!
• This unsurprisingly resulted in garbled TCP connections that subtly broke everything.
• Fix this oversight, and introduce a test that catches this kind of failure case.
  • The BoundedWriter test harness forces partial writes.
  • It also panics if the copier tries to write too much, as without this feature the (broken) copier would just loop indefinitely.
• A future commit should refactor the copier, which was written by eta in a fit of Brussels-induced overconfidence (see also f173e44c, "oops yolo"), to be a bit less prone to this sort of error.
  • trinity suggested factoring out the buffer logic, which I will go and do next!