Improve flow lookups

[pgigis]

Summary

This PR makes the flowkey protocol-aware and cleans up how the active/passive flow tables are defined. It also pre-sizes those tables from config so lookups/inserts do a little less work under load.

Testing

• cmake -S . -B build && cmake --build build
• Unit tests (ctest --test-dir build) if available
• Other (describe): Targeted sanity checks around packet parsing and traffic matching

Notes

• No new flags or config changes.
• Small internal behavior change: flows are now keyed with protocol as part of the identity, so TCP and UDP traffic with the same IP/port tuple no longer collapse to the same flow key.