Sets mode to “local”, i.e., parallelization is turned off and all necessary stuff is cleaned up.
For socket and mpi mode
parallel::stopCluster() is called.
For BatchJobs mode the subdirectory of the
containing the exported objects is removed.
After a subsequent call of
parallelStart(), no exported objects
are present on the slaves and no libraries are loaded,
i.e., you have clean R sessions on the slaves.