[DISCUSS] Offset checkpoint notification to the System Consumer

[Boris Shkolnik]

Currently we keep track of the progress of each consumer using
checkpointing mechanism. These checkpoints are controlled by Samza, and the
system consumer is not aware of it.
But some systems may require knowledge of the current checkpointed position
in the stream.
For this purpose we propose an optional notification mechanism for input
systems.
See the doc attached to SAMZA-1042 for details.