Memory leak in PDFGenerator.parallel

[pavoljuhas]

The multiprocessing Pool does not terminate when PDFGenerator is garbage collected. Repeated calls to parallel fork new processes and use all memory.

Solution: make sure the pool is terminated when out of scope.

[stevenhua0320]

@sbillinge We might make this into the next version release as this needs careful refactor on the parallel dumping logic and needs to decide what are not needed in the cycle of PDFGenerator.