Move Journey store implementations to a different repository

[mwaaas]

Descriptions

We have a journey store interface, then we have implementations of that interface here
https://github.com/ussd-airflow/ussd_engine/tree/master/ussd/store/journey_store

currently, we only have dummy store, dynamodb and YAML store for saving the journeys.
The list can grow big and it's a good idea to have a separate development for those, so separating each store to its own repository