Better memory management: Soft and hard limits for sessions

There should be a way to be informed when a session allocates more memory than a soft limit, defined by a global such as session_memory_soft_limit. When the server allocates more than this for a session, it posts a message in the error log, including details about the allocation (which ID, USER, HOST, Command, State).

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

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