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.
Passing certificates one by one instead of passing whole group is way more inefficient. But maybe it's not the case with map_async, because it does some splitting on it's own I'll check how it works.