Smartstack Scaffolding - Stack Workers

[timbeccue]

Summary

This is one of two related PRs which have been split for easier reviews. In this PR, we first add the stacking core and data model. The next PR (441) will integrate these into the pipeline.

• Add StackFrame DB model and helper functions to keep track of frames during smartstack observations
• Add stacking worker (Check redis for new subframes, stack completion check, stack timeout detection)
• Add stacking supervisor (runs a separate stack worker per camera)
• Modify run_pipeline_stages: add return value with the reduced image filepath
• unit tests

Overall, these changes are intended to get smartstacking ready with everything except the stacking algorithm itself.

One particular change I want to flag is that stage_utils.py, run_pipeline_stages used to have no return value, but now returns the images that were just processed, in order to easily pass the filepath to the stack db and worker. I don't think this causes problems in any existing calling code, but since it modifies a core banzai function I want to be sure.