Fix consistency of parallelCollectionScan to match other new cursor commands such as the aggregate command.

1. Allow passing in batchSize with command, to return firstBatch results of batchSize.
2. Return firstBatch or remove firstBatch ? I can see that returning firstBatch could easily blow through 16MB of results for the command so maybe it's not feasible ?
3. ok for each cursor should be 0/1 instead of boolean
4. return error code on illegal numCursors