"Turn on the SMTP keep alive feature" seems non-functional

Problem/Motivation

This module supplies a "Turn on the SMTP keep alive feature" feature but it does not seem as though there's a realistic scenario where the connection would be re-used. This is because the SMTP mailer object that contains the connection gets recreated any time mail is sent.

Steps to reproduce

1. Install the SMTP module and configure it appropriately.
2. Turn on your debugger and set a break point at the start of the smtpConnect() method in PHPMailer\PHPMailer\PHPMailer.
3. Make a request that will trigger multiple e-mails (I don't know off the top of my head of anything in core that would do this unfortunately).
4. Step through this function. You'll see that the (null === $this->smtp) check passes and the ($this->smtp->connected()) check fails, forcing reconnection.

Proposed resolution

Store the mailer object in the service if keep alive is enabled. I'll attach an MR for this shortly.