Multiprocess parsing of der_certs.

Parsing takes a lot of time, so I added some multiprocessing. It could be further improved by creating 1 more process that is responsible for sending certs to the pool, because right now if main process is busy the subprocessses in pool aren't completely utilized.
Did you intend to apply _scan_der_cert to each certificate individually or have the pool handle groups of certs?
The first way seems more efficient (you could do that using map_async btw)