WL#2442: Batched operation of multi UPDATE statements

Multi update functions apply their updates in a function
called multi_update::do_updates
In this function a loop of all tables to update is used
together with a loop over all references to update.

The inner loop can be batched by introducing a new handler routine
update_batch_pos(array of record[0], array of record[1], array of references,
                 array of error returned)

This function is simple to implement for those handlers not wanting to
implement this function.