Perform eviction of aggregated metrics upon full

[praveingk]

The data-path xflow (tc/xdp) ebpf program has to detect if the hashmap is full, and will need to evict entries.
Several approaches can be followed, like:

Evicting specific entries based on LRU (Least Recently Used)/ timeout.
Time-based hashmap eviction : Maintain two redundant hashmaps, and switch-over for collection.

[praveingk]

Exploring if using arrays (to maintain flowid) is more efficient in terms of throughput.