Deadlock if Stream Shutdown races with Push

[multippt]

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])
}