Use mongo `$position` modifier when adding entries to a response

[prashtx]

I believe we currently use $slice and $push to add a new entry to the beginning of a response's entries array. After upgrading to MongoDB 3.0, we can use the $position modifier to directly place the new entry at the beginning rather than end of the array.

http://docs.mongodb.org/manual/reference/operator/update/position/#up._S_position