Free wand pixel cache

[btimby]

Wand creates large temp files called pixel caches. These will fill the harddrive if unchecked. The wand library cleans up atexit. However pvs is long lived.

http://docs.wand-py.org/en/0.6.2/whatsnew/0.6.html#improved-memory-deallocation-atexit-support

Instead, we need to lock threads out of the wand library, perform shutdown, delete temp files, then re-initialize the library and unlock. R/W locking is needed.

https://pypi.org/project/readerwriterlock/