samples: net: zperf: Rewrite download part to use sockets

Rewrite the TCP/UDP download part of the zperf sample to use socket API.
For UDP, performance impact is negligible (< 1 Mbps), for TCP it's
noticable, but still throughputs can be considered satisfactory (up to
~75 Mbps).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>