When running pvpython, if your Python script loaded some plugins (from
shared libraries) and called "quit" or "exit" in the Python script
itself, pvpython would segfault, if you created a proxy from the plugin.
The problem was arising from the fact that the plugin shared library was
unloaded before the VTK object corresponding to the proxy was destroyed
during ProcessModule singleton cleanup. This was happenining because the
vtkPVPluginLoader cleaner singleton (which unloads libraries) was
finalized before the vtkPVPluginLoader singleton (which cleans up
unreleased proxies).

Fixed by adding a Schwarz counter to ensure that the plugin loader is
not finalized before the process module. Since vtkPVPluginLoader.h is not
included in a large number of translation units (and there's never need
for it), the overhead for this is minimal.

Change-Id: I0db6f4fc4e0657a623cb2dccefbb55a90ec05cab