Reorganise ingestion store classes to match architecture  #109

Is your feature request related to a problem? Please describe.
The store classes in ingestion are currently all mixed into a single storage package. This makes it difficult to see that there should be a clear distinction between serving stores, warehouse stores, and errors stores.

Describe the solution you'd like
I'd like to propose the following package structure

• feast.store.warehouse
• feast.store.serving
• feast.store.errors

feast.store, will be used for any common classes and interfaces. Naming conventions should follow a similar style to the "source" package.