Implement pool events according to mysql/mysql2 API.
according to mysql/mysql2 documentation :
|acquire||This event emits a connection is acquired from pool.|
|connection||This event is emitted when a new connection is added to the pool. Has a connection object parameter|
|enqueue||This event is emitted when a command cannot be satisfied immediately by the pool and is queued.|
|release||This event is emitted when a connection is released back into the pool. Has a connection object parameter|