Add persistent storage to store current state

[lalitpagaria]

Problem

Twitter V2 APIs, Play Store Review APIs and etc have capabilities to fetch result after tweet id, review id respectively. So idea is to store intermediate information in persistent store to avoid fetching duplicate data.

Propose solution

sqlalchemy already included in the dependencies so it would easy to add storage layer. sqlalchemy can also enable user to use their choice of data store supported by it's DBAPI.

Persistent layer would be helpful for workflow engine to recover from some failure scenarios.