This allows distinguishing pending syncs that didn't have a DELAY_REQ sent from ones that did but used a seqnum of 0, like the very first one. Specifically, if the first one or more syncs are still pending and we send the first DELAY_REQ for a later pending sync, then the DELAY_RESP would've been wrongly associated to the very first pending sync because of the seqnum. Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3383 Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6361>