To simplify progress logic, we now report progress only from the root node. This
avoids clogging MPI channels with unnecessary progress events while still giving
the application a vague idea of the progress. Cleaning up the logic ensures that
we never deliver messages to client for progress more frequent than 1 sec (in
past we always had to send 0 and 1 progress values.

Removing obsolete class vtkPVMPICommunicator. It was only needed to handle the
async mpi progress events. We don't use async messages for progress anymore.

Change-Id: I04213540ce9ac4b7338143c51d7fd479711e5ff9