Create benchmark for various forms of dictionaries

[BCSharp]

This PR is mainly done to test whether and when Hashtable in a single writer, multiple readers scenario is better than SynchronizedDictionary (#218).

Short answer: It isn't.

Longer answer: run the benchmarks yourself. The instructions are in README.md.

It looks like with every new version of .NET ComcurrentDictionary gets faster and faster. It already outpaced Hashtable. OTOH, the performance of Hashtable is static. Perhaps nobody is working on it anymore.

[slozier]

Thanks! Looks good to me. I assume you meant ConcurrentDictionary and not SynchronizedDictionary in your comment.

I merged #320 already so this will probably generate an obsolete warning and break the build?

[BCSharp]

Correct on both points. I've already updated the comment.