perfschema thread walker needs to take thread's LOCK_thd_kill to prevent
the thread from disappearing why it's being looked at.
But there's no need to lock it for the current thread.
In fact, it was harmful as some code down the stack might take
LOCK_thd_kill (e.g. set_killed() does it, and my_malloc_size_cb_func()
calls set_killed()). And it caused a bunch of mutexes being locked under
LOCK_thd_kill, which created problems later when my_malloc_size_cb_func()
called set_killed() at some unspecified point under some
random mutexes.
to iterate over all status variables one should use
LOCK_all_status_vars not LOCK_status
this fixes sporadic mutex lock inversion in plugins.password_reuse_check:
* acl_cache->lock is taken over complex operations that might increment
status counters (under LOCK_status).
* acl_cache->lock is needed to get the values of Acl% status variables
when iterating over status variables
fixes the failure of
./mtr --ps sys_vars.gtid_slave_pos_grant sysschema.v_session_ssl_status
safe_mutex: Found wrong usage of mutex 'LOCK_thd_data' and 'LOCK_active_mi'
* treat FUNC/ARRAY variables as SESSION (otherwise they won't be shown)
* allow SHOW_SIMPLE_FUNC everywhere where SHOW_FUNC is
* increase row buffer size to avoid "too short" assert
