Make Signal thread-safe

[ulex]

Current Signal implementation use a thread-unsafe LifetimedList as a backend. There are some exception-protecting mechanisms in LifetimedLists, which significantly reduce the probability of exception during Advise/Fire, but they can't protect in all cases. Since it is allowed to use arbitrary schedulers for RdSignal, reliable thread-safe solution is required here.

Sample test to violate existing implementation:

[Test]
public async Task TestSignalStress()
{
  var sig = new Signal<bool>();
  using (var run = new LifetimeDefinition())
  {
    var lt = run.Lifetime;
    var sw = Stopwatch.StartNew();
    var fireTask = Task.Run(() =>
    {
      while (sw.ElapsedMilliseconds < 500 && lt.IsAlive) 
        sig.Fire(true);
    }, lt);

    Parallel.For(0, 100000, i =>
    {
      using (var ld = new LifetimeDefinition())
        sig.Advise(ld.Lifetime, x => { });
    });

    await fireTask;
  }
}