ENH: relax processorField waiting requirements

- for interface polling previously required that both send and recv
  requests were completed before evaluating (values or matrix update).
  However, only the recv needs to be complete, which helps disentangle
  the inter-rank waiting.

  NB: this change is possible following (1f5cf395) that replaced
      UPstream::resetRequests() call in favour of UPstream::waitRequests()