The background is that one user had a lot of views and using some complex
queries on information schema temporary memory of more than 2G was used.

- Added new element 'total_alloc' to MEM_ROOT for easier debugging.
- Added MAX_MEMORY_USED to information_schema.processlist.
- Added new status variable "Memory_used_initial" that shows how much MariaDB
  uses at startup. This gives the base value for "Memory_used".
- Reuse memory continuously for information schema queries instead of
  only freeing memory at query end.

Other things
- Removed some not needed set_notnull() calls for not null columns.