Add multithreading support in attack functions

[unicornsasfuel]

Certain attack functions such as Vaudenay's padding oracle attack or the multi-byte XOR solver would benefit from some parallelization, but currently this isn't done.

[unicornsasfuel]

So discussions with people more intimately familiar with Python than I suggest that multi-threading in Python is effectively useless except if you're trying to overcome limitations based on network latency or some such.

Considering that, multi-threading in any active attacks based on oracles would be useful, but local attacks like the multi-byte XOR attack would not benefit from multi-threading.