speed up dc recovery as don't spend time on pickling/unpickling data

MR for fix #556 When there are lots of keys to recover in final_merge pickle dump consume lots of memory and CPU as it's perform checking already pickled objects (https://docs.python.org/2/library/pickle.html#relationship-to-other-python-modules "The pickle module keeps track of the objects it has already serialized, so that later references to the same object won’t be serialized again. marshal doesn’t do this.")
Use cat to concatenate files
We can make the same with a dump files: in the processes pool generate per range dump files and combine them there. In the future, I want to try to get rid of pickled objects and its files by iterating without meta-data (which is much faster) and use only key files.