Previously, the reactor would use an `UnboundedSender` to send things to
the `RawCellStream`, in order that the reactor wouldn't block if you
failed to read from the latter. This is bad, though, since it means
people can just run us out of memory by sending lots of things.

To fix this, we make the new `StreamReader` type (which does the reading
parts from `RawCellStream`) keep track of the stream's receive window
and issue SENDMEs once *it* has consumed enough data to require it, thus
meaning that we shouldn't get sent enough data to fill the channel
between reactor and `StreamReader` (and, if we do, that's someone trying
to flood us, and we abort the circuit).

As hinted to above, the `RawCellStream` was removed and its reading
functionalities replaced by `StreamReader`; its writing functionalities
are handled by `StreamTarget` anyway, so we just give out one of those
for the write end. This now means we don't need any mutexes!

note: this commit introduces a known issue, arti#230