network: terminate Hive connect goroutine on Stop

[janos]

This PR addresses the issue where Hive connect goroutine does not exit after calling Hive.Stop(), creating goroutone leaks. It appears that it was expected for Hive.ticker.C to be closed after Hive.ticker.Stop() is called, but that is not the case. Ticker channel is not closed leaving the for loop waiting indefinitely as no more ticks are sent.

This issue can be reproduced with this network ad hoc test:

func TestConnectLeak(t *testing.T) {
	s, pp, err := newHiveTester(NewHiveParams(), 1, nil)
	if err != nil {
		t.Fatal(err)
	}
	err = pp.Start(s.Server)
	if err != nil {
		t.Fatal(err)
	}
	pp.Stop()

	stackSymbol := []byte("github.com/ethersphere/swarm/network.(*Hive).connect")
	for i := 0; i < 100; i++ {
		buf := make([]byte, 1<<16)
		runtime.Stack(buf, true)
		if !bytes.Contains(buf, stackSymbol) {
			return
		}
		time.Sleep(10 * time.Millisecond)
	}
	t.Error("connect goroutine did not exit")
}

[zelig]

oops nice catch