The aim here is to get to a stage where we have a `PersistEventStore` that holds all the write methods used during event persistence, so that we can take that class out of the `DataStore` mixin and instansiate it separately. This will allow us to instansiate it on processes other than master, while also ensuring it is only available on processes that are configured to write to events stream.
This is a bit of an architectural change, where we end up with multiple classes per data store (rather than one per data store we have now). We end up having:
1. Storage classes that provide high level APIs that can talk to multiple data stores.
2. Data store modules that consist of classes that must point at the same database instance.
3. Classes in a data store that can be instantiated on processes depending on config.
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.
