Per-channel update count since the last _DeltaSnapshot was written.
Maps channel name → number of supersteps that wrote to this channel
since its last snapshot blob. Used by pregel.create_checkpoint to
decide when to write the next snapshot (when the count reaches the
channel's snapshot_frequency, snapshot fires and the count resets
to 0). Absent on threads that don't use delta channels. Version-format
independent — works for int, float, and string version schemes.
