ENH: bundle broadcasts (#2371)

- additional Pstream::broadcasts() method to serialize/deserialize
  multiple items.

- revoke the broadcast specialisations for std::string and List(s) and
  use a generic broadcasting template. In most cases, the previous
  specialisations would have required two broadcasts:
    (1) for the size
    (2) for the contiguous content.

  Now favour reduced communication over potential local (intermediate)
  storage that would have only benefited a few select cases.

ENH: refine PstreamBuffers access methods

- replace 'bool hasRecvData(label)' with 'label recvDataCount(label)'
  to recover the number of unconsumed receive bytes from specified
  processor.  Can use 'labelList recvDataCounts()' to recover the
  number of unconsumed receive bytes from all processor.

- additional peekRecvData() method (for transcribing contiguous data)

ENH: globalIndex whichProcID - check for isLocal first

- reasonable to assume that local items are searched for more
  frequently, so do preliminary check for isLocal before performing
  a more costly binary search of globalIndex offsets

ENH: masterUncollatedFileOperation - bundled scatter of status