Port ConditionalWeakTable from .NET Core 3.x to .NET Standard 2.0

As per LUCENENET-610, the WeakDictionary that has been created to support FieldCache and a few other Lucene.NET features does not perform well enough in highly concurrent environments.

The ConditionalWeakTable<TKey, TValue> is a suitable replacement on .NET Standard 2.1, but on .NET Standard 2.0, it doesn't expose the enumerator or the AddOrUpdate method. All of Lucene.NET's usages require one or the other.

So, it would definitely be worth the effort to port the full implementation to .NET Standard 2.0.

This port should be added to J2N in the J2N.Runtime.CompilerServices namespace (to match .NET's namespace convention).