Handle error codes 3xxx

[renecannao]

Historically, up to MySQL 5.6:

• error codes 1xxx were server errors
• error codes 2xxx were client errors
  In MySQL 5.7 things changed, as error codes 3xxx were introduced.

As ProxySQL handles all error codes > 2000 as client errors, it assumes that the connection to the backend is broken. Although this is correct in MySQL 5.6, isn't correct anymore with MySQL 5.7

[ovaistariq]

+1

[renecannao]

This is only partially implemented. There are still few cases that need to be handled.