- the case of 'fan-like' processor was previously assumed to be
  rare (see merge-request !487 and issue #2084).

  However, Vaggelis Papoutsis noticed that even fairly normal geometries
  can trigger problems.

- replaced the old patch/patch matching style with a more general
  edge-based synchronisation and matching that appears to handle
  the corner cases inherently. The internal communication overhead
  is essentially unchanged, and the logic is simpler.