ID service doesn't shut down properly

[marten-seemann]

Running this test with -count=1000 leads to a massive Go routine buildup:

func TestHostClose(t *testing.T) {
	host, err := libp2p.New(libp2p.ListenAddrStrings("/ip4/127.0.0.1/tcp/0"))
	require.NoError(t, err)
	require.NoError(t, host.Close())
}

From the pprof:

goroutine profile: total 707
701 @ 0x46c0f6 0x47d312 0xfaf4a5 0x49df21
#	0xfaf4a4	github.com/libp2p/go-libp2p/p2p/protocol/identify.(*ObservedAddrManager).worker+0x344	/home/marten/src/go/src/github.com/libp2p/go-libp2p/p2p/protocol/identify/obsaddr.go:258