Make use of Ecstatic.Store configurable and extensible

[sikanrong]

Currently Ecstatic.Store.Ets is used directly in the code in several places. Instead we should use a wrapper module such as Ecstatic.Store which internally defers to a configurable storage module.

In this way, the user can implement with their own storage backend by implementing the Ecstatic.Store @behaviour in a given module, and then configuring that storage backend using Mix.Config