Not many companies eat Cisco Systems ' (CSCO) lunch, but F5 Networks (FFIV) is one of them. F5 pummeled Cisco in a fast-growing part of the market for data center gear. Now, F5 Networks finds itself the hunted. Seattle-based F5 holds 47% of the market for application delivery controllers. ADCs manage Internet traffic in data centers, routing workloads among servers. Citrix Systems (CTXS) holds 18% of the market and Cisco, the No. 1 maker of