nice project and well written.

Pool offer the same functionalities ( which is nice, mysql and mysql2 doesn't ! ).
But there is no option not in internal pool, and internal pool has some additional benefits :

• reliability: when a connection is released, transaction state is checked ( completely reset even )
• efficiency: validation is on or off. default internal pool won't validate connection if used a few millisecond before, and that change drastically the time of one query. ( and a marginal thing : query will create a double promise when a single one if enough)

So not this time, but the comparison is always nice (and coding style is better, give me some ideas for improving design)