Tune socket buffer size(s)

[jmpesp]

tokio::net::TcpSocket offers both set_recv_buffer_size and set_send_buffer_size.

Tune these away from their defaults and investigate the impact on performance. This may also necessitate changing other things like the maximum number of in-flight jobs.

[leftwo]

Yeah, the flow control stuff between upstairs and downstairs I think will be effected directly by this.