Lighter processing for OCR activities

When running OCR, we sometimes have issues because processing is "too heavy":

• use 2 or 3 Go of disk space for a one page PDF created by erp5_document_scanner, because we convert pdf -> png -> tiff before sending to tesseract. Modern Ghostscript supports running tesseract directly, so we use it if it's available.
• use 300% of CPU. Fixed by setting OMP_THREAD_LIMIT when running tesseract. This will only apply when OCR from Images. OCR embedded in Ghostscript does not seem to need this.
• ... and often crash, so is restarted. This is fixed by updated tesseract.

Updates of ghostscript and tesseract are part of https://lab.nexedi.com/nexedi/slapos/merge_requests/985