perf: support IMIX benchmarking

Example of usage (run benchmark with packet sizes of 84, 256 and 1280 bytes,
with probabilities of 10%, 15% and 75%):

./perf/ipsec_perf --cipher-algo aes-docsis-128 --hash-algo docsis-crc32 \
                  --job-size 84,256,1280 --imix 10,15,75 --job-iter 3000000 \
                  -o 0 --arch AVX512