Investigate using alternative ConcurrrentHashSet implementation

There is a lower level implementation of ConcurrentHashSet available on NuGet that supports all of our target platforms:

https://github.com/i3arnon/ConcurrentHashSet/blob/master/src/ConcurrentHashSet/ConcurrentHashSet.cs

The only limitation is that it does not actually implement ISet<T>. I don't believe that will be a problem, though. But, since this collection is used directly in IndexReader it is worth looking into.