There was a race condition in the way relfilenodes were chosen, because
QE nodes chose relfilenodes for existing relations, e.g. at REINDEX or
TRUNCATE, independently of the master, while for newly created tables,
the relfilenode was chosen by the master. To fix:

1. If the OID of a newly-created table is already in use as relfilenode
   of a different table in a QE segment, use a different relfilenode.
   (This shouldn't happen in the dispatcher, for the same reasons it
   cannot happen in a single-server PostgreSQL instance)

2. Use a small cache of values recently used for a relfilenode, to close
   a race condition between checking if a relfilenode is in use, and
   actually creating the file