scans wait for ... before sending END_SCAN
@sole has a use case where an external process wants to add something to the ScanNode.info
dict in Redis so that it is saved by the Nexus writer when it finalizes scan writing. Of course this means that the scan in Bliss should wait for this external process to say "yep I'm done, you can send the END_SCAN event now". I seem to remember there was already a mechanism foreseen for this where the scan waits for something external to happen. Perhaps with beacon channels @sebastien.petitdemange ?