--temp-pool: change default to 0 and deprecate

According to benchmarks made by Axel (and further analysis) , this code behind --temp-pool=1 in create_tmp_table() causes a heavy mutex contention:

  if (use_temp_pool && !(test_flags & TEST_KEEP_TMP_TABLES))

    temp_pool_slot = bitmap_lock_set_next(&temp_pool);

The --temp-pool option was implemented to address some bottleneck in Linux kernel, which is reported to be fixed nowadays.

In MySQL, --temp-pool was deprecated in 5.7 and removed in 8.0.

Let's do the following:

• Set --temp-pool=0 by default in MariaDB-10.5 (currently its default is 1)
• Mark it as deprecated in mysqld --help output
• Remove it in some later version, tentatively in 10.6.

In the meanwhile, we should try to find proves that this problem was really addressed in Linux.