EXPLAIN FORMAT=JSON: support join buffering

- Basic support for JOIN buffering
- The output is not polished but catches the main point:
  tab->select_cond and tab->cache_select->cond are printed separately.
- Hash join support is poor still.

- Also fixed identation in JOIN_TAB::save_explain_data

mysql-test/r/explain_json.result

@@ -309,4 +309,37 @@ EXPLAIN
   }
 }
 drop table t1;
+#
+# Join buffering
+#
+create table t1 (a int, b int);
+insert into t1 select A.a+10*B.a, A.a+10*B.a from t0 A, t0 B;
+explain format=json
+select * from t1 A, t1 B where A.a=B.a and A.b < 3 and B.b < 5;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "A",
+      "access_type": "ALL",
+      "rows": 100,
+      "filtered": 100,
+      "attached_condition": "(A.b < 3)"
+    },
+    "block-nl-join": {
+      "table": {
+        "table_name": "B",
+        "access_type": "ALL",
+        "rows": 100,
+        "filtered": 100,
+        "attached_condition": "(B.b < 5)"
+      },
+      "buffer_type": "flat",
+      "join_type": "BNL",
+      "attached_condition": "(B.a = A.a)"
+    }
+  }
+}
+drop table t1;
 drop table t0;