Exiting python

I got another issue during my test; when I import arrayfire then want to exit, python becomes hung:

```
>python
Python 3.6.3 (v3.6.3:2c5fed8, Oct 3 2017, 18:11:49) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import arrayfire
>>> ^Z
```

After Ctrl+Z the used memory from ~600MB gets to ~250MB and stops there, no cpu on it.

Reproduce the report with Python 3.6.3 on Windows by importing arrayfire and then sending Ctrl+Z at the interpreter prompt. Investigate the shutdown path for the hang and memory remaining after exit; done means Python exits normally after the import.

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