Rewrite TripleBuffer as a true lockless, flipping, triple buffer

[smoogipoo]

What made me do this is over the last couple of days I encountered an assert in song select using a debug o!f:

Unhandled exception. NUnit.Framework.AssertionException: : 
   at osu.Framework.Logging.ThrowingTraceListener.Fail(String message1, String message2) in /Users/smgi/Repos/osu-framework/osu.Framework/Logging/ThrowingTraceListener.cs:line 27
   at System.Diagnostics.TraceInternal.Fail(String message, String detailMessage)
   at System.Diagnostics.Debug.Fail(String message, String detailMessage)
   at osu.Framework.Allocation.TripleBuffer`1.getPendingReadBuffer() in /Users/smgi/Repos/osu-framework/osu.Framework/Allocation/TripleBuffer.cs:line 93
   at osu.Framework.Allocation.TripleBuffer`1.GetForRead() in /Users/smgi/Repos/osu-framework/osu.Framework/Allocation/TripleBuffer.cs:line 65
   at osu.Framework.Allocation.TripleBuffer`1.GetForRead() in /Users/smgi/Repos/osu-framework/osu.Framework/Allocation/TripleBuffer.cs:line 75
   at osu.Framework.Platform.GameHost.DrawFrame() in /Users/smgi/Repos/osu-framework/osu.Framework/Platform/GameHost.cs:line 514
   at osu.Framework.Threading.GameThread.processFrame() in /Users/smgi/Repos/osu-framework/osu.Framework/Threading/GameThread.cs:line 451

It can be repro'ed via:

[Test]
public void StressTest()
{
    TripleBuffer<object> tripleBuffer = new TripleBuffer<object>();

    new Thread(() =>
    {
        while (true)
        {
            using (tripleBuffer.GetForWrite())
            {
            }
        }
    }).Start();

    new Thread(() =>
    {
        while (true)
        {
            using (tripleBuffer.GetForRead())
            {
            }
        }
    }).Start();

    Thread.Sleep(100000);
}

Which is pretty scary. Either one of the two asserts in getPendingReadBuffer() can trigger. We've also been seeing some intermittent CI failures though I can't point to one immediately (happens once in a blue moon), likely related to this.

Separately from the above, I became unconvinced of the existing implementation while reading through the code. As for the immediate issue above, I can't figure out why it's happening, but even more - I've been saying for a long time now that I want TripleBuffer to evenly distribute occupancy across all buffers. I wrote a test (TestSceneTripleBufferOccupancy) to test this and found that the existing implementation breaks down almost as soon as both producers/consumers aren't running at absolute unlimited rate:

Current	New

So instead of fumbling around with the existing implementation, I chose to reimplement it hopefully following the existing principles of triple buffering in graphics: a front/back buffer with an intermediate flip buffer.
The write buffer is flipped onto the flip buffer after the write completes, and the flip buffer is flipped onto the read buffer when a read is requested. This is also completely lockless now, compared to the current implementation which locks between the two threads.

Note that this is a custom implementation so I'm not sure if this is exactly how this is supposed to be implemented, but it makes sense in my head and seems to work in practice, so... /shrug?