Replace O(n) linear search through NAT table with O(1) reverse lookup map
for reply packet NAT translation.
Changes:
- Add reverseConnKey type for reverse NAT lookups
- Add reverseNatTable map to ProxyHandler for O(1) lookups
- Populate both forward and reverse maps when creating NAT entries
- Replace iteration-based reverse lookup with direct map access
Performance:
- O(n) → O(1) complexity for reverse NAT lookups
- Eliminates lock-held iteration on every reply packet
- Removes string comparisons from hot path
- Expected 10-50x improvement for reverse NAT lookups
This addresses Critical #1 from performance analysis where reply path
was walking the entire NAT table to find original mapping.
