Add noCursorTimeout option to command cursors

The NoCursorTimeout bit in the OP_QUERY header prevents the server from closing a cursor that's idle for more than 10 minutes. Clients can set this bit if they may spend more than 10 minutes processing a batch of results.

The "aggregate" and "parallelCollectionScan" commands should allow the client to turn off cursor timeouts, too. The current syntax for specifying a cursor returned from aggregate is:

{

  aggregate: "collection",

  pipeline: [ ... ],

  cursor: { batchSize: 1000 }

} 

I propose adding:

{

    cursor: { batchSize: 1000, noTimeout: true }

}

Perhaps "timeout: false" is clearer, though less consistent with the query API.

"parallelCollectionScan" could include the option at the top level of the command spec:

{

  parallelCollectionScan: "collection",

  numCursors: 4,

  noTimeout: true

}

Prompted by this question:

http://stackoverflow.com/questions/24997404/cursor-not-found-when-using-parallel-scan-of-pymongo