See #1994

Setting `closed` on the socket (which can mean either "the socket is broken"
or "we should close the socket at the next opportunity") was racing against
the final write. Even if socket.closed is true, we should attempt to send
buffered message data to the socket, before we send the `finalData` and
actually call `Close`.

* fix a race condition: a call to `Write` does not spawn a writer goroutine
  if the trylock is held, so `BlockingWrite` must check for fresh data after
  releasing the trylock
* streamline some close/finalize logic

* Placate `go vet`
* Reorder the `send` loop, clarify things a little

This is a gotcha called out in the `time.After` docs. `time.After` will
leak the underlying channel if nothing ever receives on it.

- get rid of command channel in Client
- get rid of loginTimer; use other timers instead
- move debugging code to debug.go