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
This patch removes original dumping keys. @agend please provide alternative way of this functionality. It could be made as I described above: each subprocess from pool generates per range dump file and at the final merge main process combine these dump files in one.