Close and reinitialize Mysql handle before reconnection

[haoqixu]

We can not reuse a MYSQL handle without re-initialization after mysql_close(). We should close and reinitialize the handle before reconnection.

• https://dev.mysql.com/doc/c-api/5.7/en/mysql-close.html
• https://mariadb.com/kb/en/mysql_real_connect/

Fix #527

[charlesfg]

@haoqixu, is this problem related to reconnecting after any failure between the sysbench client and MySQL?

[haoqixu]

@charlesfg yes. sysbench fails to reconnect to the mysql/mariadb after failure. more details can be found in #527