feat/mocking fs tests

[yeqown]

• tests: using afero to abstract file system so that I can code unit test cases for file operation related function
• tests: more test cases for db and db_compact; also fix some bugs here;
• tests: add compact interval option; add test case for startCompactRoutine

[sweep-ai[bot]]

Apply Sweep Rules to your PR?

• [ ] Apply: All new business logic should have corresponding unit tests.
• [ ] Apply: Refactor large functions to be more modular.
• [ ] Apply: Add docstrings to all functions and file headers.