p2p/simulations: encapsulate Node.Up field so we avoid data races

The Node.Up field was accessed concurrently without "proper" locking.
There was a lock on Network and that was used sometimes to access
the  field. Other times the locking was missed and we had
a data race.

For example: https://github.com/ethereum/go-ethereum/pull/18464
The case above was solved, but there were still intermittent/hard to
reproduce races. So let's solve the issue permanently.

resolves: ethersphere/go-ethereum#1146

swarm/network/simulation/simulation_test.go

@@ -124,7 +124,7 @@ func TestClose(t *testing.T) {
 
 	var upNodeCount int
 	for _, n := range sim.Net.GetNodes() {
-		if n.Up {
+		if n.Up() {
 			upNodeCount++
 		}
 	}
@@ -140,7 +140,7 @@ func TestClose(t *testing.T) {
 
 	upNodeCount = 0
 	for _, n := range sim.Net.GetNodes() {
-		if n.Up {
+		if n.Up() {
 			upNodeCount++
 		}
 	}