Note:

• This PR is going into the develop branch rather than feature/session-manager as it doesn't hurt
  to merge it into develop and reduces the diff size of the feature branch.
• No functional change is introduced by this PR.

📚 Context

In the feature/session-manager branch, we're working towards improving the way that we handle
websocket disconnects. As part of this work, we'll need to have the final steps of AppSession
cleanup (done by the shutdown() method) happen when a session is garbage collected rather
than when the browser's websocket connection is dropped.

To make this easier, this PR splits out file changed handler registration/deregistration into two helper
methods. We also add a __del__ method to AppSession, which calls the shutdown() method on
garbage collection.

Finally, we add a test to make it harder to introduce AppSession-related memory leaks related to file
watchers holding references to AppSession file changed handler methods. This exact issue has
caused memory leaks in the past, so we should be extra-careful to avoid it given that our work to
defer AppSession cleanup is particularly prone to running into this.

• What kind of change does this PR introduce?

  • Refactoring
  • Other, please describe: Prep work for upcoming feature

🧪 Testing Done

• Added/Updated unit tests