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.
IIUC if the certificate passes DER parsing in non-strict mode then the else clause in line 28 will be invoked but not this else clause, so the checks will not be applied to the parsed certificate.
Unless the certificate fails non-strict DER parsing, in which case you don't even end up with a valid Certificate object, these checks should be applied.