Handling done in red_channel instead of per channel, using call backs
for the channel specific part.
Intended to reduce furthur reliance of channels on RedChannel struct.
The commit makes the code harder to understand because of the artificial
get_serial stuff, should later be fixed by having a joint migration
header with the serial (since all channels pass it).
For ussage in the send_item callback. It's only valid during this
time anyway (should make it return NULL in other occasions?)
No more direct usage of RedChannel.send_data.marshaller by channels.
This is stylish change again. We are talking about a RedStream object,
so let's just name the variable "stream" everywhere, to avoid
confusion with a non existent RedPeer object.
https://bugs.freedesktop.org/show_bug.cgi?id=34795
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.
