mongoimport --mode=upsert doesn't do bulk upsert

bulk upsert is much faster than the current mode of inserting one at a time.

if i'm not using upsert mode i can injest much faster using mongoimport.

if i turn on upsert mode it goes much slower than it should.

suggested fix: pull _id out of incoming documents and use that as the bulk upsert's selection portion.

alternately enable a server side replace option that handles dup key error by replacing the doc in mongo instead of favoring what is already there. more like sql replace into rather than update/insert (because what i really want is replace into and i don't need fancy selectors to update documents on non-_id keys).