Move `StopSignal` from `SuperSpreader` to `TrackBallast`

[benjaminoakes]

Background

In a discussion with @codenamev, we decided that StopSignal is a better fit for TrackBallast than SuperSpreader now that TrackBallast exists.

Historical Note

StopSignal was added to SuperSpreader in January 2023, about a year ago, because it was originally written for use with SuperSpreader. Since that time, we have started using StopSignal separately from SuperSpreader.

Proposed

• Add TrackBallast.redis like SuperSpreader.redis
• Add the StopSignal class and its spec to TrackBallast
• Document Redis setup in the README
• Consider implementing issues relating to StopSignal
  • Document StopSignal super_spreader#62
  • Extend StopSignal to allow for groups of jobs to be stopped super_spreader#63
• Move any unimplemented issues relating to StopSignal to https://github.com/doximity/track_ballast/issues

Note

StopSignal is not planned to be included in the first production-ready release of TrackBallast, but is intended to be added soon after that ships

[benjaminoakes]

This has been merged to v0.2.0. Releasing v0.2.0 will complete this issue.