add atomic output collection to DBQuery

A Map/Reduce query can atomically output its results to a separate collection, which is nice for a job where you want to create "views" of the data that you can then query within.

It would be nice if immediate queries could do the same thing, eg.

db.mycollection.find(

).out(attachcollection);

It could operate asynchronously, and attachcollection would remain unmodified until the query was complete after which it would be atomically swapped with the temporary collection in which the new results reside.