network: randomize peer iteration order

While iterating over map in getPeers() is non-deterministic it's not really
random enough for our purposes (usually maps have 2-3 paths through them), we
need to fill our peers queues more uniformly.

Believe it or not, but it does affect performance metrics, baseline (four
nodes, 10 workers):

RPS ≈  7791.675 7996.559 7834.504 7746.705 7891.614 ≈ 7852   ±  1.10%
TPS ≈  7241.497 7711.765 7520.211 7425.890 7334.443 ≈ 7447   ±  2.17%
CPU %    29.853   39.936   39.945   36.371   39.999 ≈   37.2 ± 10.57%
Mem MB 2749.635 2791.609 2828.610 2910.431 2863.344 ≈ 2829   ±  1.97%

Patched:

RPS    8180.760 8137.822 7858.358 7820.011 8051.076 ≈ 8010   ± 2.04% ↑ 2.01%
TPS    7819.831 7521.172 7519.023 7242.965 7426.000 ≈ 7506   ± 2.78% ↑ 0.79%
CPU %    41.983   38.775   40.606   39.375   35.537 ≈   39.3 ± 6.15% ↑ 5.65%
Mem MB 2947.189 2743.658 2896.688 2813.276 2863.108 ≈ 2853   ± 2.74% ↑ 0.85%

pkg/network/server.go

@@ -1146,6 +1146,9 @@ func (s *Server) iteratePeersWithSendMsg(msg *Message, send func(Peer, bool, []b
 	if len(peers) == 0 {
 		return
 	}
+	mrand.Shuffle(len(peers), func(i, j int) {
+		peers[i], peers[j] = peers[j], peers[i]
+	})
 	pkt, err := msg.Bytes()
 	if err != nil {
 		return