Write unit tests for Postgres package

[vincent6767]

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.

[harlow]

thanks for the pr 💯