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 poll 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.
Shouldn't `err` always be defined here :)?