Parent issue: #1181

Create a new workspace package for the core tracker module: src/core.

• Package folder: ./packages/tracker-core-lib
• Crate name: bittorrent-tracker-core-lib

This would allow other projects to use the same core tracker logic and I would make harder to pollute the core tracker with higher level login like web frameworks, etc.

Database migration are only used inside the core tracker module, so they have to be moved into the new package.

cc @da2ce7