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
I inspect code and can't find place which use dump file. Now as we don't iterate through file we can remove it. Add special option for program to iterate through file, but this will slowdown performance. What will be best for this case? Do we need this dump file?