Better memory management: Soft and hard limits for Mariadb Server

There should be a way to be informed when Mariadb server allocates more memory than a soft limit, defined by a global such as server_memory_soft_limit. When the server allocates more than this, it posts a message in the error log, including details about the allocation (what pool requires more memory) and the current allocation of each memory pool and sessions.

There should be a way to stop Mariadb server from allocating more memory than a hard limit, defined by a global such as server_memory_hard_limit. In the event that a Mariadb server attempts to exceed this limit, it would be prevented with an error visible to all clients and posted in the error log.

Each of these limits can be disabled with a value of 0 (default).