make bufwriter log the packets it writes
no more need for this
make sure conn.getRedo() can't hang waiting for values from conn.In
cleanup testing code

* there was a bug with one of the numSeen/numAccept checks
  so i rewrote the thing in a cleaner way, supporting concurrency,
  using topic.  these kinds of checks are still hard to get right,
  and this version is arguably overengineerd.  but it works.
  at least the api is simpler and it's a bit easier to understand
  the interactions.
* implement "what have you seen" logic properly, using a diff of
  sorted data. and use it, in favor of counting how many lines seen.