[OrionSink] Check the events have effectively been persisted before killing the channels on Cygnus exit

[frbattid]

On Cygnus exit, when the last event is taken from a channel it seems that channel may be closed. Nevertheless, there exists the possibility the event is reinjected by the sink into the channel because of a problem with the backend. In that scenario, closing the channel is not correct, and some mechanism must be created in order to know the event has effectively been persisted (Flume does not provide any ACK mechanism from the sink to the channel, for instance).

Effort: 2 man day