BUG #13831. Cleaning up progress reporting in ParaView.

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
Merge topic '13831_fix_progress' into next

e1f6e4e BUG #13831. Cleaning up progress reporting in ParaView.