ENH: consolidate PstreamBuffers reduced communication bookkeeping

- can reduce communication by only sending non-zero data (especially
  when using NBX for size exchanges), but proper synchronisation with
  multiply-connected processor/processor patches (eg, processorCyclic)
  may still require speculative sends.

  Can now setup for PstreamBuffers 'registered' sends to avoid
  ad hoc bookkeeping within the caller.