Send acks without getting masters involved

Currently the acks (acknowledgement) from the collector are sent back via the master(s). In case the master is down, the acks will be lost. It seems that the even the multi masters cannot handle this single-point-failure issue.

A re-design of sending the acks back without getting the master involved will be very beneficial. The single-point-failure can be removed to some extent, and the efficiency of the acks will also be improved since the master is not a bottleneck any more.