Clearing Pool doesn't remove event listeners

[marek-ganko]

Event listeners are not removed when the connection is closed and pool.clear() is triggered.

I'm using typeorm SapDriver, and when the connection is opened and then closed over 10 times (in tests) than I'm getting this warning:

MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 poolError listeners added to [EventEmitter]. Use emitter.setMaxListeners() to increase limit

Also it seems that PoolManager.eventEmitter() shouldn't be a static method, because in case of multi-tenants scenario, errors will be shown for every tenant connection.