[migration] Better implementation for 0-byte files, cf. #697

This implementation (unfortunately) introduces one more CLI, but
it makes the 0-byte files migration fully independent from the rest.
The main reason is that the query to extract them from the CASTOR
Nameserver is pretty expensive.
The README.md file and the packaging have been updated accordingly.

cta.spec.in

@@ -334,7 +334,8 @@ directory metadata into the EOS namespace.
 %attr(0644,root,root) %{_bindir}/begin_vo_export_to_cta.sh
 %attr(0644,root,root) %{_bindir}/export_production_tapepool_to_cta.sh
 %attr(0755,root,root) %{_bindir}/tapepool_castor_to_cta.py
-%attr(0755,root,root) %{_bindir}/complete_tapepool_export.py
+%attr(0755,root,root) %{_bindir}/zerolen_castor_to_cta.py
+%attr(0755,root,root) %{_bindir}/complete_cta_export.py
 %attr(0644,root,root) %{_bindir}/vmgr_reenable_tapepool.sh
 %attr(0644,root,root) %{_bindir}/cta-catalogue-remove-castor-tapes.py
 %attr(0644,root,root) %config(noreplace) %{_sysconfdir}/cta/castor-migration.conf.example