Patches to algorithm for download from CASTOR

This patch changes the algorithm for downloading archived .tgz files from CASTOR. The combined size of these files for a given production ID is no longer a problem as the required disk space decreases to about the size of the largest .tgz file. Moreover, a race condition with the external xrdcp process was found and a patch is included to deal with such cases. A 10% tolerance on the number of requested XML logs is taken when unpacking archived productions in order to account for XML files which may prove corrupted upon parsing at a later stage.