json解析函数key中带有特殊关键字，比如（# . |）会出现无法解析的情况

[413331861]

测试例子如下：

MySQL [test]> select * from a;
+------+-----------------------------------+
| id | data |
+------+-----------------------------------+
| 2 | {"text.abc":"MXCHIP won a prize"} |
| 4 | {"text|abc":"MXCHIP won a prize"} |
| 1 | {"text_abc":"MXCHIP won a prize"} |
| 3 | {"text#abc":"MXCHIP won a prize"} |
+------+-----------------------------------+
4 rows in set (0.01 sec)

MySQL [test]> select get_json_string(data,'$.text_abc') from a;
+---------------------------------------+
| get_json_string(data, '$.text_abc') |
+---------------------------------------+
| NULL |
| NULL |
| MXCHIP won a prize |
| NULL |
+---------------------------------------+
4 rows in set (0.01 sec)

MySQL [test]> select get_json_string(data,'$.text.abc') from a;
+---------------------------------------+
| get_json_string(data, '$.text.abc') |
+---------------------------------------+
| NULL |
| NULL |
| NULL |
| NULL |
+---------------------------------------+
4 rows in set (0.01 sec)

MySQL [test]> select get_json_string(data,'$.text#abc') from a;
+---------------------------------------+
| get_json_string(data, '$.text#abc') |
+---------------------------------------+
| NULL |
| NULL |
| NULL |
| NULL |
+---------------------------------------+
4 rows in set (0.01 sec)

MySQL [test]> select get_json_string(data,'$.text|abc') from a;
+---------------------------------------+
| get_json_string(data, '$.text|abc') |
+---------------------------------------+
| NULL |
| NULL |
| NULL |
| NULL |
+---------------------------------------+
4 rows in set (0.01 sec)

[morningman]

Using boost 'tokenizer' instead of 'split' to parse the json_path parameter.
And implement the json_path_prepare function to parse the json path only once instead of per each row.