Better Pipe task scheduling to avoid extra collision

To avoid extra processing costs due tasks that are running and being rolled back by contention, it would be nice to have a way to spread (over time) pipes to the same sink.

To do so, we'll probably need to take a look at this [Appengine API](https://cloud.google.com/appengine/docs/java/javadoc/com/google/appengine/api/taskqueue/RetryOptions).
