To avoid connection leak, call `close()` immediately after connection
is established. In regular tree pool, unhealthy connections are handled
by background goroutine which calls `redialIfNecessary()` to reestablish
connection. Here it is not viable so connection must be close.

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

@@ -0,0 +227,4 @@
+	routinesBefore := runtime.NumGoroutine()
+	for i := 0; i < 1000; i++ {
+		_, err = treePool.AddNode(context.Background(), AddNodeParams{CID: cnr})
+		require.NoError(t, err)

@@ -0,0 +42,4 @@
+		ni := apinetmap.NodeInfo{}
+		ni.SetAddresses(server.lis.Addr().String())
+		ni.SetPublicKey(server.key.PublicKey().Bytes())
+		err := nodes[i].ReadFromV2(ni) // no other way to set address field in netmap.NodeInfo