feat: refactor file storage to fix thread safety issues

[falconandy]

Summary

Refactored file storage to fix thread safety issues.

1. Added unique part to file names (timestamp, file manager identifier) to eliminate possible collisions between different file manager instances.
2. Changed internal file format to use json-event-per-line structure instead of single-line-with-json-event-array.
3. Added a few unit tests to cover file manager logic.

Checklist

• [x] Does your PR title have the correct title format?
• Does your PR have a breaking change?: No

[justin-fiedler]

Resolved in #181