If a table has no unique indexes, write set key information will be collected on all columns in the table.
The write set key information has space only for max 3500 bytes for individual column, and if a varchar colummn of such non-primary key table is longer than
 this limit, currently a crash follows.
The fix in this commit, is to truncate key values extracted from such long varhar columns to max 3500 bytes.
This may potentially lead to false positive certification failures for transactions, which operate on separate cluster nodes, and update/insert/delete table rows, which differ only in the part of such long columns after 3500 bytes border.
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>