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.
nit: rename to reporter_workers, to reflect the name of the module (since flags are global it helps if the flag is easily associated with the module)