cluster generator should not parse the date field

There's no point parsing the Date field before using it as part of the hash in the 'cluster' generator.

Two messages with different Date fields are not the same message, even if the converted dates are the same. Parsing/conversion hides such differences, as well as being unnecessary work.

Locate the cluster generator and trace how the Date field is handled before contributing to the message hash. Verify the behavior with messages whose raw Date fields differ but convert to the same date; done means those differences remain distinguishable without unnecessary date parsing.

Written by the indexing model from the issue text.