Fix deadlock if Stream Shutdown races with Push.

[multippt]

Fixes #191

There's a deadlock scenario if the stream is being shutdown while still receiving events, which can occur if using LifecycleEvents and cancelling the context.

Example reproduction case

func TestStreamShutdown(t *testing.T) {
	s := NewStream(1, 2)
	events := testEvents(1)
	s.Shutdown()
	s.Push(events[0])
}