Use PikePDF and split into pages before feeding to Wand
- Removes PyPDF2 as a dependency
- Instead uses pikepdf for image extraction
- Feeds only single pages to Wand instead of full PDFs
Memory usage for a celery worker is ~100 MB when idle, and 150-250 MB while processing a PDF (either PikePDF or Wand). After processing it returns to 100 MB.
Fixes #322 (closed)