Add Option to Disable Strelka Backend Shutdown

[weslambert]

Describe the change
This change provides the option to prevent strelka-backend from shutting itself down after max_files or time_to_live has been reached.

Describe testing procedures
Rebuilt the strelka-backend image and tested using a value of 0 for max_files and time_to_live in backend.yml

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[phutelmyer]

Tested configurations with 0 max_files and 0 time_to_live successfully. I advise caution as those limits (specifically time) were implemented to reduce the potential memory issues. However, this implementation works and I appreciate you putting the time in.