split-clients: new tool to split large PCAP into multiple smaller ones

This is essentially the opposite of limit-clients.lua.
This new script assigns one client IP address to a single output and
copies all packets for single client into the same output file.

Client assignment is pseudorandom and does not guarantee uniform
assignment. Clients typically do not generate equal load in any metric
(be it QPS or overall resources required to process client's query),
so there would be little point in complicating code for theoretical
uniformity.

Stats generated at the end provide some insight into uniformity of the
split on per-client and per-packet basis.