Instead we use LOCK_status only to protect summary of thread statistics and use a new mutex, LOCK_show_status
to protect concurrent SHOW STATUS.

sql/mysqld.cc:
  Add LOCK_show_status
  Don't free LOCK_status while calculating status variables.
sql/mysqld.h:
  Add LOCK_show_status
sql/sql_show.cc:
  Use LOCK_show_status to protect SHOW STATUS instead of LOCK_status.