samples: net: zperf: Optimize configuration for better performance

Increase the number of network packets and buffers for better TCP
performance in the sample out-of-the-box.
Decrease the network buffer data size for better buffer management in
the sample (less buffer space wasted for L2 header). The only drawback
of this is reduced TCP TX performance, but less than 2 Mbps in my case.
Finally, enable speed optimizations for another small performance boost.

As the RAM requirements of the sample now increase considerably in the
default configuration, add a note in the readme file about it, and how
to make it fit into smaller boards.

Tested on nucleo_h723zg:
  Before (current defaults):
    UDP      TX          RX
         76.47 Mbps  93.48 Mbps
    TCP      TX          RX
         76.18 Mbps  67.75 Mbps

  After (new defaults):
    UDP      TX          RX
         76.08 Mbps  93.62 Mbps
    TCP      TX          RX
         74.19 Mbps  85.51 Mbps

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