BUG: prevent blocking when processor directories are missing (fixes #2027)

- ensure that the processor directory cache is consistently available
  by providing appropriate synthesized names. If the expected files
  really do not exist, it will still fail at a later point (but not in
  the caching).

  This addresses cases that have been manually distributed
  (different content on different nodes, but the same root path),
  while still maintaining the readDir on master only for massively
  parallel cases (#1946). It also addresses regular cases with
  gaps in the processor directories.