Concurrency Tests and Thread-Safety

[Firionus]

TODO

• [x] Patch Thread Safety
  • [x] Test Immutability of Maps returned by Getters
  • [x] Create kinda reliable concurrent tests
  • [x] Make tests pass by locking
  • [x] refactor tests for readability and clarity
• [x] SubscriptionHandler Thread Safety
• [ ] Client-Server-based Tests
• [x] Document Locking Strategy and Locking Orders
• [x] check TODOs

[Firionus]

started development on branch thread-safety