wrong pool 'connection' event object

from https://github.com/mariadb-corporation/mariadb-connector-nodejs/issues/342

When a user registers a pool.on('connection') event handler that throws, the pool enters an infinite connection creation retry loop, creating far more connections than connectionLimit allows and leaking TCP connections.

additionally : the returned connection object is the raw connection object, without the callback or promise wrapper, making it not usable, so conn.query will throw an error