Send larger row batches over the wire

Our on-the-wire row batch size is the same as the in-memory size (1024 rows by default). It might make sense to increase the wire-size to reduce the RPC-per-row overhead, and decrease context-switching in the receiver.

KRPC makes it quite natural to do that: each row batch can be serialized as a sidecar in memory-size batches. The receiver can then read each batch in turn as though it were sent individually, without any need to stitch together (or split up) serialized batches.