EXPLAIN FORMAT=JSON: further development

Writing JSON:
- Fix a bug in Single_line_formatting_helper
- Add Json_writer_nesting_guard - safety class

EXPLAIN JSON support
- Add basic subquery support
- Add tests for UNION/UNION ALL.

sql/sql_explain.h

@@ -112,6 +112,8 @@ public:
 
   int print_explain_for_children(Explain_query *query, select_result_sink *output, 
                                  uint8 explain_flags, bool is_analyze);
+  void print_explain_json_for_children(Explain_query *query,
+                                       Json_writer *writer, bool is_analyze);
   virtual ~Explain_node(){}
 };