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.
I did some additional testing, and apply_async is actually faster. It's because there is no point in additional splitting of already small batches coming from log client.