The query generated by anonymise_issue_history is unecessary complex and
can be very slow on large DB.
With this patch we will execute one query per patron instead.
See patch "Bug 19599: Simplify the query" on the bug report for an
alternative.
Signed-off-by: Nick Clemens <nick@bywatersolutions.com> Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com> Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>