First iteration of a structure for configuring feature flags.

This allows us to explicitly set some flags during testing to enable new and upcoming features. It helps us test the features before releasing them, without having to change source code. I'd like to unify the interface for this. It's intentionally kept very simple.

For now this includes just one flag. I'd like to add a flag for using the new immutable ID tracker once merged.

In debug builds, all flags are enabled by default (as per config/development.yaml). In chaos testing we'll probably enable all features by setting the appropriate environment variables.

I chose to use this global structure because it turns out to be extremely cumbersome to propagate the flags to all places we might use it. I want to prevent having to propagate it (adding a lot of changes), having to undo all changes one or two versions later when we remove the flag. While I normally hate a global/static structure, I think this is a fair exception.

Let's use this PR as a place to discuss this feature.

All Submissions:

• Contributions should target the dev branch. Did you create your branch from dev?
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests?
2. Have you formatted your code locally using cargo +nightly fmt --all command prior to submission?
3. Have you checked your code using cargo clippy --all --all-features command?