ParaView relied on using Py_NewInterpreter() for creating separate pyhton
interpreters in the application. Py_NewInterpreter() causes issues with Python
threading (esp. GIL related things). As a result we decided to refactor Python
embedding in ParaView/VTK (VTK too since the introduction on Matplotlib).

The new design uses a global Python interpreter (used by all filters including
Matplotlib rendering code) with the ability to create interactive interpreter
(for the PythonShell).

On disconnect, we only discard the interactive interpreter, thus the global
Python interpreter remains active. We will cleanup servermanager and simply
Python Modules in subsequent commits to work correctly with long live Python
interpreters.

Change-Id: I705b024542e2598ca55d62d32de7e7d704f48e79