Deprecate spider_semi_table_lock and spider_semi_table_lock_connection

We change the default value of the plugin variable, spider_semi_table_lock, to 0 (no semi-table lock) and then deprecate the variable. Setting the variable to 1 (semi-table lock) would result in poor performance and it is rare to use non-transactional engines these days. Further, maintaining the variable complicates the code.

The variable, spider_semi_table_lock_connection, should be too deprecated because it is for tweaking the semi-table locking.

See https://mariadb.com/kb/en/spider-server-system-variables/ for the details of the variables.