High CPU usage in open_tables() at high concurrency

During a HammerDB TPROC-C run on 192 physical cores, open_tables() was the largest MariaDB CPU hotspot.

Overhead Shared Object Symbol

8.96% mariadbd open_tables(THD*, DDL_options_st const&, TABLE_LIST**, unsigned int*, unsigned int, Prelocking_strategy*)

4.45% mariadbd mtr_t::commit_log_release()

4.31% mariadbd mtr_t::finish_writer<false>()

3.15% mariadbd ssux_lock_impl<false>::rd_lock_nospin()

open_tables() consistently consumes approximately 9% of total CPU, making it the largest single hotspot during the workload.

The attached perf-report.txt confirms the result and shows open_tables() being called through the normal SQL statement preparation path:

Sql_cmd_dml::prepare()

-> open_tables_for_query()

-> open_tables()

Branch tested was bb-12.3-av-perf-3 (12.3.3). As MDEV Optimize open_tables to take O(N) time MDEV-30182 is related the commits were cherry-picked onto bb-12.3-av-perf-3 and confirmed not to bring benefit for this workload.