Fix plugin exit when transport is SMTP

[pawelangelow]

Problem: The plugin does not exit correctly when the transport is SMTP.

Why:
Triggering the fastify.nodemailer.close(done) is expected to
start disposing of resources procedure. However the mailer implementation
proxies the execution to the underlying SMTP transport.
In order to exit properly (after the plugin successfully closed),
the callback has to be called on the plugin event close.

[giovanni-bertoncelli]

Thank you, I've also opened an issue #107 but probably @lependu is not maintaining this anymore. Using this package should be avoided.