key: Add an access benchmark with a bigger map.
key: Add an access benchmark with a bigger map.

This benchmark shows how the performance of GetFromMap() becomes awful
due to its O(N) behavior with composite keys.

Change-Id: Ib4e32474e755e35984a3810da5d09b82cfdcb636

aristanetworks/goarista,aristanetworks/goarista