Support JSON format tables

Since HIVE-19899, Hive supports JSON format tables officially. It'd be good to add the same functionality for Impala.

Subtasks can be:

• Support loading metadata of JSON format tables. Make statements that don't need to read data files work first, e.g. DESCRIBE, SHOW CRETATE TABLE, SHOW PARTITIONS, etc.
• Support creating JSON format tables by "STORED AS JSONFILE" clause.
• Support reading JSON format tables. All SELECT queires should work.
• Support writing to JSON format tables.