Function pump 

pub fn pump<S: Sink>(
    reactor: &mut Reactor<Wire<S>>,
    batch: &mut Vec<Cmd>,
) -> usize

Run everything that is framed, in batches, and write the replies.

The inline driver: it is what a caller who is already on the shard thread uses in place of the loop, and it goes through the same two walks the loop goes through (15 section 7). batch is the caller’s, so a driver in a hot loop hands the same Vec back every time and never allocates.