4.x: Improve and fix CompositeDisposable

[akarnokd]

This PR cleans up the internal logic of CompositeDisposable by avoiding some allocations and making sure publicly visible state is available in a thread-safe manner.

The main safety offender was the IsDisposed reading the _disposed plain field without a barrier. Assuming the .NET memory modell is compatible with the C++ 11/Java memory model, such reads could be cached and the reader may never see _disposed turn true:

var composite = new CompositeDisposable();

handOff(composite);

while (!composite.IsDisposed) Thread.SpinWait();

// resume

without a barrier, this could end up like this:

var isDisposed = composite.IsDisposed;
while (!isDisposed) Thread.SpinWait();

and never complete.