refactor: add image scheduler abstraction

[edparris]

This change introduces an abstraction for an ImageScheduler that is responsible for selecting the next image to process from a collection of pending requests. Instead of reading directly from the image request queue the ModelRunner loops will ask the ImageScheduler for the next image whenever there are no regions to process. The current code has been refactored as a first-in-first-out (FIFO) scheduler which will let us maintain the current behavior using the new abstraction. Future PRs will introduce new scheduling options based on lessons learned from the ongoing load testing.

New unit tests were added to verify behavior of the FIFO scheduler and the ModelRunner updates. In addition these updates were deployed into a development account and integration test images were run to confirm basic function was not impacted.

Checklist

Before you submit a pull request, please make sure you have the following:

• Code changes are compact and well-structured to facilitate easy review
• Changes are documented in the README.md and other relevant documentation pages
• PR title and description accurately reflect the changes and are detailed enough for historical tracking
• PR contains tests that cover all new code and the code has been manual tested
• All new dependencies are declared (if any), and no unnecessary libraries are added
• Performance impacts (if any) of the changes are evaluated and documented
• Security implications of the changes (if any) are reviewed and addressed
• I have read the Contributing Guidelines and agree to follow the Code of Conduct

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[drduhe]

LGTM