Packages a batch from the Screen operator to send to its
user connection. In the original Drill, that connection was always a
network connection, and so the outgoing batch is serialized to a set
of buffers ready to send. However, the REST server runs in the same process.
The original REST query implementation serialized the data to buffers, then
copied the data to a big buffer to be deserialized, causing significant memory
pressure. This version allows the user connection to elect for serialization,
or just to access the original source batch.