samples: net: zperf: Fix sample performance issues

Sockets service thread need to be cooperative for best performance.

Enable CACHE_MANAGEMENT as some boards don't do it by default, it has
devastating impact on the performance as well.

Finally, increase FDS count, so that uploads still work when both TCP
and UDP receiver are enabled.

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