Prevent MEMORY tables from consuming all RAM

While the size of a single MEMORY/HEAP table is limited by max_heap_table_size there is no limit for the number of tables (max_tmp_table variable exists, but has no function and is deprecated) or for the total amount of RAM available for MEMORY tables.

Memory usage of implicit memory tables is more or less limited by MIN(tmp_table_size, max_heap_table_size)*max_connections (when not taking subqueries and stored function calls into account), but so far nothing prevents a single user session from explicitly creating MEMORY tables until the mysqld process runs into an out-of-memory (OOM) situation ... which could e.g. be used for denial-of-service attacks ...