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.
Remove all uses of @end in the marshaller, instead just using
the C struct array-at-end-of-struct. To make this work we also remove
all use of @end for switches (making them C unions).
We drop the zero member of the notify message so that we can avoid this
use of @end for a primitive in the marshaller (plus its useless to send
over the wire).
We change the offsets and stuff in the migration messages to real pointers.
