Duplicate message detection is not good enough: different messages with
the same unicity value may bear different serialization_tags. This code
does not takes this into account, which can lead to deleting such tagged
message and validate an untagged one, which breaks serialization_tag
contract of preventing any further activity validation until execution
of all such-tagged validated activities is successful.
Also, it is not validation's node job to deduplicate: it can happen during
message execution without slowing down this crucial (performance-wise)
activity node.
As a result, distribute methods of SQLDict and SQLQueue can be factorised.