c++ client expose option for client-side batching

[emaxerrno]

currently we flush() to the socket with ever RPC. This makes the RPC low latency but we give up all the throughput.

We could have a timeout style check. if not every 10RPC's or every 10ms? (smallest increment of the low resolution time?) kinda a bad deal for latency sensitive workloads but probably amazing for throughput ones.

See if exposing this is not too much work.