Fix parsing of row events for MySQL8 partitioned table

[dongwook-chan]

Since MySQL 8.0.16, partition info has been added into extra_data in row event.

Current code is aware of extra_data but doesn't parse it accurately. The program reads extra_data_length / 8 bytes from packet instead of correct byte-length extra_data_length - 2. This causes the library to skip parsing of extra_data.

In some cases exceptions are raised, and in other cases parse result shows incorrect field values as in issue #354. (undefined behavior)

below are the modifications made:

1. Correct byte-length of extra_data to conform to replication protocol
   extra_data_length / 8 -> extra_data_length - 2

   • AS-IS:
     https://github.com/noplay/python-mysql-replication/blob/3de6ff499f7695a800409341f4f859cac5b724d0/pymysqlreplication/row_event.py#L60
   • TO-BE
     
     Refer to MySQL Document and MySQL source code for correct bytes-to-read.

2. Parse extra_data according to extra_data layout to get partition id ( or ndb info)

        +----------+--------------------------------------+
        |type_code |        extra_row_ndb_info            |
        +--- ------+--------------------------------------+
        | NDB      |Len of ndb_info |Format |ndb_data     |
        | 1 byte   |1 byte          |1 byte |len - 2 byte |
        +----------+----------------+-------+-------------+

        In case of INSERT/DELETE
        +-----------+----------------+
        | type_code | partition_info |
        +-----------+----------------+
        |   PART    |  partition_id  |
        | (1 byte)  |     2 byte     |
        +-----------+----------------+

        In case of UPDATE
        +-----------+------------------------------------+
        | type_code |        partition_info              |
        +-----------+--------------+---------------------+
        |   PART    | partition_id | source_partition_id |
        | (1 byte)  |    2 byte    |       2 byte        |
        +-----------+--------------+---------------------+

3. data type tests were added to test parsing of extra_data, and correct value of prtition_id