Sending emails should not block the main request but be sent async

[Ambient-Impact]

The SMTP server can be slow, like several seconds slow, which holds up the response from Drupal when submitting a form or performing an action that has to send an email, such as a user with elevated roles logging in. We'll need to be able to send mail asynchronously after the response is sent to the browser.

Symptoms

• Logging in as a user with elevated roles takes a good few seconds, likely because the SMTP server connection is holding up the request until emails have been sent.
• admin/reports/status is slow to load every time, likely because the SMTP module always runs a connection test via its hook_requirements() every time.
• admin/config is also really slow to load, very likely because the status checks are run every time if a user has access to view them, and the SMTP module's connection check is run again as described above.

Requirements

1. Sending mail needs to not delay sending the response to the browser.
2. Sending mail needs to happen as quickly as possible, so that time sensitive emails arrive in a reasonable amount of time, ideally not noticeably delayed compared to sending synchronously; for example, a password reset email is expected to arrive within 30 seconds or less from submitting the form, so we can't rely on waiting for cron to run to do this. See The quest for a more intelligent queue system that supports prioritizing, parallel processing, etc. #3

Solutions

It looks like the Queue Mail module might be just what we need to satisfy requirement 1, but it doesn't seem to concern itself how the queue is processed, which means that requirement 2 is something we'll need to solve ourselves or find a module/library to handle that.

Symfony Mailer

Additionally, the Drupal Symfony Mailer module (see #1) doesn't support queuing (though Symfony Mailer itself does), and the Queue Mail module doesn't support the Drupal Symfony Mailer module because ugh, so we may have to contribute that if no one else does by the time we get to this. Edit: See Symfony Messenger details in #3