Include new flag to prevent import duplicated files and fail import if conflict

Add this new feature. Currently the etp-server includes everything and overwrites objects that are already in a given dataspace.

With a new flag (--reject-duplicates) we will be able to ignore files that are already there and reject the import if any incoming has conflicts. Rules:

Same GUID and same date: just ignore - already in db Same GUID and diff date: conflict - reject import Different GUID: normal behavior