creating ttl index on history colls

At this point, it wouldn't be completely necessary, as the function is only used in createCollection, which is already wrapped in _logOperation. On the other hand, I found it less confusing as all the functions of MongoDatabase are wrapped in _logOperation (it basically shouldn't do any evil), and if just the _createTTLIndex was used without any other functions, then the operation would have to be logged. But back again, this function is yet private ... let's discuss this.