Sending several "KILL QUERY" statements to target a connection running
"SELECT SLEEP" could freeze the server.

The locking order in Item_func_sleep was wrong and this could lead to a
dead lock.

This patch solves the issue by resolving the locking order properly.


sql/item_func.cc:
  - Moved LOCK_user_locks critical region so that it doesn't share space with
    mysys_var->mutex region; this can lead to deadlock.