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
Iterating withoug metadata is quite simple - just send a flag saying you do not want timestamps, that could be a single command line option.

What about this patch?
It replaces a loop over unpickled objects, which are then pickled into output file, with a simple copy of multiple files into outpug one, is it ok?