More fuzzing.

Signed-off-by: Corbin Simpson <cds@corbinsimpson.com> <simpsoco@google.com>
Implement endpoint improvements discussed on IRC.

In particular, instead of using a callback to set up connections, use promises
to deliver the fount and drain. No changes to tube library code were needed.

Signed-off-by: Corbin Simpson <cds@corbinsimpson.com> <simpsoco@google.com>