Clean up client/server transport shutdown

[dfawley]

From #4447:

We should cleanup the transport code later (when? 🤷‍♂️).

• the channel (controlbuf) is closed by the reader (loopyWriter()) instead of the writer (reader()) (hahahahaha)
  • which is, counterintuitive
• the multiple caller of t.Close() is probably part of the cause that RPCs tend to fail with transport is closing
  • if the transport is only closed by the reader(), and the other places just tells reader() to close, it might make things cleaner