Set NOTIMEOUT flag in Mongo queries

By default, Mongo will close a cursor after 10 minutes of inactivity. When you try to read from a closed cursor Mongo will raise `cursor not found on server` (as in foursquare/twofishes#4).

Setting NOTIMEOUT flag will prevent Mongo from closing it. However, if the indexer crashes and a cursor isn't exhausted you should close it manually.