onClose hook hangs indefinitely

Hi,
I'm reporting a bug (that took nearly 2 years to be found..). It seems that onClose hook of this plugin is not implemented correctly since close() function on nodemailer transport does not expect any callback, you can see that clearly on this row: https://github.com/nodemailer/nodemailer/blob/master/lib/smtp-transport/index.js#L407. This results in the fastify process to hang indefinitely without any chance to terminate anytime. I can make a PR for this.