SWEEP: Lucene.Net.Store: Fixed several Dispose() methods so they are safe to be called multiple times

[NightOwl888]

Fixes #841. Part of #265.

This adds an atomic CompareAndSetIsOpen(bool expect, bool update) method to BaseDirectory and BaseDirectoryWrapper that can be used to ensure only a single call to Dispose(bool) will succeed. This is the same method signature as J2N.Threading.Atomic.AtomicBoolean and can be used the same way (without the extra heap allocation).

protected override void Dispose(bool disposing)
{
    if (!CompareAndSetIsOpen(expect: true, update: false)) return;

    // Dispose unmanaged resources
    if (disposing)
    {
        // Dispose managed resources
    }
}

This fixes NRTCachingDirectory (as well as all other directories) so they don't throw exceptions when Dispose() is called multiple times.

This of course means that EnsureOpen() will be active immediately when Dispose() is first called, so be mindful of its usage when the Dispose() method needs to call local methods to clean up.