Add configurable global concurrent request limiting

- Add max_concurrent_requests config option (default: 1000)
- Implement global AtomicUsize counter for concurrent request tracking
- Return status 41 'Server unavailable' when limit exceeded
- Proper counter management with decrements on all exit paths
- Add comprehensive config validation (1-1,000,000 range)
- Update documentation with rate limiting details
- Add unit tests for config parsing
- Thread-safe implementation using Ordering::Relaxed

This provides effective DDoS protection by limiting concurrent
connections to prevent server overload while maintaining
configurability for different deployment scenarios.

BACKLOG.md

@@ -0,0 +1,3 @@
+- remove the CLI options, everything should be only configurable via config file
+- seperate tests into unit/integration and system tests, at least by naming convention, but perhaps there is a rust way to do it
+- add a system test which tests that the server really responds with 44 before 41