after 633417308f (MDEV-37312) lookup_handler is locked with F_WRLCK,
because it may be used for deleting rows.
And lookup_handler is locked with F_WRLCK after prune_partitions(),
but the main handler is locked before, and might expects all
partitions to be in the read least, non-pruned.
Let's prepare the lookup handler before prune_partitions().