dialect.mysql.connection-manager: handleDisconnects

I've asked on #2448 if disconnect handling should be part of **sequelize** core, but i've discovered a ghosted pool config called [handleDisconnects](https://github.com/sequelize/sequelize/blob/master/lib/dialects/abstract/connection-manager.js#L10), so I guessed it should be.

Should fix #2448 and maybe #2381 on mysql dialect.

I can't run tests on my computer, waiting for travis.
In the code i linked `this.pool` does not refer to an instance of generic pool but to an object which stubs some of its method. you should add destroy and createResource methods which checks if the connection belongs in the read or write pool and invoke the correct pool