update_parameters::prefetch_data is used for some list updates (which
need a read-before-write to determine the key to update) and for
LWT compare-and-swap. Currently they use a custom structure for
representing a read row.
Switch to the same structure that is used in evaluation_inputs (and
in SELECT statement evaluation) to the expression machinery can be reused.
The expression representation is irregular (with different fields for
the keys and regular/static columns), so we introduce an old_row
structure to hold both the clustering key and the regular row values
for cas_request.
A nice bonus is that we can use get_non_pk_values() to read the data
into the format expected by evaluation_inputs, but on the other hand
we have to adjust get_prefetched_list() to fix up the type of
the returned list (we return it as a map, not a list, so list updates
can access the index).
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