bin: forward segment-start like segment-done if parent is also a bin, fixes #575598.
Bin collects segment-start messages and segent-done messages. it posts a segment-done message to its parent, once it has received a segment-done for each segment-start. Imho it should also send a segment-start if it receives the first segment start and if parent is !=NULL. This is needed for bins in bins, so that also higher order bins can group segment-starts and segment-dones. Right now higher order bins will post a segment-done for each segment-done received.