An in-memory store. For tests, and for a dev server that should boot without a database.
It exists for a second reason: a behaviour with one implementation is a
behaviour nobody has checked. Having Postgres and ETS both satisfy
Pixelex.Store is what proves the seam is real, and it is where an adapter
author looks first.
Bounded by Pixelex.Config.max_buffer/0 — it drops the oldest events past
that. Losing history in a dev store is correct; running a laptop out of
memory is not.