Reduce plugin logging to debug level

[colin-grant-work]

What it does

At the moment, on startup and sometimes on shutdown, we log a separate message for various lifecycle events for every single plugin in the application. That's a lot of logs, most of which are completely uninformative. This PR converts them to debug level logs so that they are still available if they're of interest but won't be displayed routinely.

How to test

1. Start the application.
2. Everything should work as normal.
3. There should be a couple hundred fewer lines in your log.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍