Parallelize file processing

[kissgyorgy]

We want to run the processing in a multiprocessing.pool.Pool.

In order to simplify the implementation, and for measuring things, we would do this in 2 steps:

1. We can run unblob in for every new process_file call.
2. We can look into yara.match callbacks and start the handling in the pool right away. (For the current code, this would need more complex changes.)

[kissgyorgy]

Only after #100. We don't know if this would worth the cost of running on more memory or more CPU if we don't measure.

[kissgyorgy]

It has been implemented in #181