Connection close during long query (more than 1min) with bellow message.
```
MongoError: server <host>:<port> sockets closed]
name: 'MongoError',
message: 'server <host>:<port> sockets closed' }
```

this message come from
```
at Socket.<anonymous> (........./node_modules/mongodb/node_modules/mongodb-core/lib/connection/connection.148)
at Socket.g (events.js:180:16)
at Socket.emit (events.js:95:17)
at TCP.close (net.js:466:12)
```

It seems like that the connection closed by mongodb simply.
However, this problem does not met on 1.4 driver and mongoshell with same query.

There are no server log associating with this problem.

I can find server operation by currentOp after close connection.

There is strong reproducibility.