General Changes

• Add histogram() function.
• Optimize CASE expressions on a constant.
• Add basic support for IF NOT EXISTS for CREATE TABLE.
• Semi-joins are hash-partitioned if distributed_join is turned on.
• Add support for partial cast from JSON. For example, json can be cast to array<json>, map<varchar, json>, etc.
• Add implicit coercions for UNION.
• Expose query stats in the JDBC driver ResultSet.