This adjusts the way that TestingStream is implemented to allow for
two, separate buffers for each of the two directions. In the prior
implementation, if you called `write` and then `read`, you would
`read` the data you just wrote. Which is not what you want; you want
to block until you get data back from the other side.
I believe these were introduced previously to solve a problem that we're
no longer dealing with; specifically, if I remember correctly, we
introduced these to deal with how we were going to implement a trait.
However, they don't appear to be necessary any more, so we're going to
get rid of them, so we won't need to deal with them any longer.
This induces an added `Send` in the Network trait and its
implementations, but provides us the ability to use standard functions
with obvious extensions. So that's nice. I've also added some additional
testing to sanity check the conversions.
