app/crypto-perf: support IMIX

app/crypto-perf: support IMIX

Add support for IMIX performance tests, where a distribution
of various packet sizes can be submitted to a crypto
device, testing a closer to a real world scenario.

A sequence of packet sizes, selected randomly from a list of packet
sizes (with "buffer-sz" parameter) with a list of the weights
per packet size (using "imix" parameter), is generated
(the length of this sequence is the same length as the pool,
set with "pool-sz" parameter).

This sequence is used repeteadly for all the crypto
operations submitted to the crypto device (with "--total-ops" parameter).

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>