To avoid buffering large amounts of data in the process, we set the
worker channel size to `64`. Keep in mind that this is one
`ChannelEvent`, not one byte. `ChannelEvent::Data` can already contain
an arbitrary amount of data via its `Vec<u8>`, but in practice the
maximum is 8192, due to the use of `io::copy`.
We also remove an unused function, and move the flushing to the
`ChannelFlushWriter`.