fix: fix server multithreading

[edwinkys]

Purpose

This PR solves the issue where the server doesn't acccept multiple connections/requests at once. It will throw connection refused error or even sometimes crash the server.

Approach

1. I modified the server in a way to prevent direct mutation to the server by using Arc and Mutex for shared values.
2. I fixed the tokio::spawn code which is the primary source of this problem.
3. I remove the Server.serve() function and replace it with a standalone serve function that will handle the server and thread creation. In this function, the server is wrapped by Arc to allow threads to refer to the same server.

Testing

• I have tested this PR locally.
• I added tests to cover my changes, if not applicable, I have added a reason why.

I have modified the test suite to accomodate the changes that I made to the server functionality.

I tested this PR by using a custom Python script to create a multi-threaded request to the server while the server is running on localhost port 3141. Without this changes, the server will throw an error when encountering multi-thread requests.

Chore checklist

• I have updated the documentation accordingly.
• I have added comments to most of my code, particularly in hard-to-understand areas.