Got Mongo xunit acceptance tests passing

I don't like the change I had to make to get the acceptance tests to pass with the existing schema. It sorts on the "_id" property instead of CommitStamp. the "_id" property consists of <StreamIdGuid>/<CommitSequence>. This is slower than sorting by commit stamp.

Another option is to add an extra CommitSequence property to the document, then sort by CommitStamp, then by CommitSequence. This would be a schema change. Existing databases would still work. However, it would return the commits out of sequence if 2 commits were committed at the same time and they don't have CommitSequence populated. This is the way it's working now, so I wonder if this would be a better approach.