major refactor plan

[PhotonQuantum]

There's been a while since we touched the codebase. I believe there's a need to refactor the codebase and solve some of the current problems.

My plan for refactoring:

• General code cleanup and better documentation (General codebase cleanup #61)
• Migrate from rocket to actix-web (rocket is a one-person project, and it has a history of stalled development) (refactor: migrate from rocket to actix-web #62)
• Add unit tests to ensure the functional correctness of new code

Merged for now. Let's see if the new implementation is correct.

• Redo task queueing so it won't block incoming requests and by the way, solves per-upstream queue #24

[skyzh]

Overall LGTM, thanks for that!

[PhotonQuantum]

Actix-web based implementation has been deployed to Zhiyuan. If everything goes well, we may deploy it to Siyuan and set the logging level to warn next month.