Swappable Locking Implementation

[JakenVeina]

Added a shared/reusable implementation for multi-locking within stream operators, I.E. being able to process upstream notifications and downstream notifications at the same time, with different locks, while still preserving notification order.

I've been testing this strategy for locking within the new .Filter() operators I'm working on. Usage of this class in that case would look like....

private void OnNext(IChangeSet<T> changes)
{
    using var @lock = SwappableLock.CreateAndEnter(UpstreamSynchronizationGate);
    
    foreach (var change in changes)
    {
        ...
    }
    
    var downstreamChanges = ChangeSet<T>.Empty;
    if (_downstreamChangesBuffer.Count is not 0)
    {
        downstreamChanges = new ChangeSet<T>(_downstreamChangesBuffer);
        _downstreamChangesBuffer.Clear();
    }
 
    if ((downstreamChanges.Count is not 0) || !_suppressEmptyChangeSets)
    {
        @lock.SwapTo(DownstreamSynchronizationGate);
        
        _downstreamObserver.OnNext(downstreamChanges);
    }
}

[dwcullop]

Why would you need this? Is it as an optimization?

[JakenVeina]

Why would you need this? Is it as an optimization?

Yes. Nothing specifically more than that.

[RolandPheasant]

When I .wrote DD, I was looking for something like this but never got it quite right. Looking forward to seeing it in action