Only count the number of documents in the filesCollection if the index

The slow count performance of TokuMX was causing the C# Driver to perform when badly when inserting large amounts of data into GridFS, as the driver was doing a count every time it was ensuring the index was in place. I implemented the suggestion here in this discussion:

https://groups.google.com/forum/#!topic/tokumx-user/FcbYoFugNFM