This simplifies things as it is, but will also allow us to change the
way we traverse topologically without having to update the way push
actions work.
for the email and http pushers rather than trying to make a single
method that will work with their conflicting requirements.
The http pusher needs to get the messages in ascending stream order, and
doesn't want to miss a message.
The email pusher needs to get the messages in descending timestamp order,
and doesn't mind if it misses messages.
