As we work towards introducing consumer groups to the repository we need a more generic name for the persistence layer for storing checkpoints and leases for given shards.
* Rename `checkpoint` to `store`
Changes:
* I add postgres_databaseutils_test.go for containing all utilities for mocking the database.
* I put appName as a required parameter passed to the New() since it is needed to form the namespace column name.
* I add GetMaxInterval() since it might be needed by the consumer and the test.
* I move the SQL string package variables directly into the function that need it so we could avoid maintenance nightmare in the future.
