Refactor and mock JQ physical OPs

• Add method close() to interface DBSourceLayer (and all of its implementations)
• Create JQSourceLayer which is an implementation of DBSourceLayer that uses the Spark SQL Java API to query JSON data sources using SQL. It is a refactored version of JQDatabaseHolder. This refactoring effectively separates domain logic from DB calls.
• Fix typo in error message in PhySOLROperator