use utf8mb4 for exchange with server

current implementation for utf8 use : https://github.com/9EOR9/mariadb-connector-python/blob/master/src/mariadb_connection.c#L337

2 issues :

• default communication with the server has to use utf8mb4, not utf8mb3.
• there is normally no need of this additional query to set communication charset. That has to be set with greeting packet.