hold_item called on init_send_data, matching release.
This is not the behavior of red_worker - we ref++ (==hold_item) when
sending the item, and --refs when releasing it, instead of only holding
if the send is blocked.
Note 1: Naming: hold_pipe_item is the proc name, the variable is called
hold_item, this is similar to release_item/release_pipe_item naming.
Note 2: All channels have empty implementation, we later use this when
red_worker get's RedChannelized.
