EDIT2 : Ok so i found a solution, i use min_sync_delay to initialize syncat ! =D But i still got one second delay between the time i send the command and the time the photos is taken. Is it normal ?
Some delay is needed, but unless you have a lot of cameras it shouldn't need a second. The way synced commands work is they schedule something to happen at a certain time in the future, so all the command can be sent to all the cameras and they can be ready to shoot at the same time. This trades delay for better synchronization between cameras. The more cameras you have, the more delay will be needed.
I think you figured this out, but to use a synchronized command, you have to run mc:init_sync() once after the cameras are connected, and then send the sync time as an option to mc:cmd for each command you want synced. For example:
mc:cmd('shoot',{syncat=500})
would schedule all the cameras to shoot 500ms in the future. Of course, the delay has to be long enough for all the cameras to get the command and be ready. min_sync_delay might not be enough.
It's worth noting that in some windows configurations, init_sync will behave very badly because it only gets time at 15ms resolution. This should be obvious in the output of init_sync, the "send" lines will all be 0 or 15.
If you don't care about having the cameras all shoot at the same time, you could just loop through the list of cameras and send each one a shoot command in turn.
I'm not sure how you are doing you hdr command or what your needs are, but if you want each shot to be synchronized and the time of each exposure accurately recorded, then you probably want to do something like testshots, where the individual shoots are synchronized, rather than the entire hdr sequence. In that case, you should be able to record the shot time pretty accurately by logging the current time + syncat just before you send the shoot command (there is some delay that isn't accounted for in the sync process, but it shouldn't be more than tens of milliseconds).
If you want to post your modified source, I can try to look at it, but no promises.