lib-sieve: plugins: duplicate: Only save duplicate entry when the last script executed successfully.

This prevents prematurely marking duplicates when errors occur during result
execution. Before, the pending duplicate database changes were committed after
each script executed in the sequence. If a later script failed, duplicate tests
would mark duplicate messages prematurely.

Disadvantage is that actions in earlier scripts that are shielded by a duplicate
test can be executed twice erroneously when errors occurred in later scripts in
the previous delivery. Still, unexpected duplicate action execution is better
than losing mail due to erroneous duplicate detections.

Fixing this properly requires extensively restructuring result execution.