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.
Would be handy to have the exception  information contain the log entry index so we know which log entry caused the parser to blow up.
Add a TODO (to address in a follow-up pull request!) to store this information and report it as an observation.