Increase the 4mb BSON Object Limit to 16mb

Mostly for tracking who/how many others are interested in this, but it would be nice to have the option of >4MB objects.

My specific use case is the storage of Twitter social graph data. It's not too much of an issue at the moment as it takes about a million id's to overflow the limit, but it's a "nice to have" to not have to hack up some other solution.