Reducing memory when using information schema

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.

include/my_alloc.h

@@ -43,6 +43,7 @@ typedef struct st_mem_root
   /* if block have less memory it will be put in 'used' list */
   size_t min_malloc;
   size_t block_size;               /* initial block size */
+  size_t total_alloc;
   unsigned int block_num;          /* allocated blocks counter */
   /* 
      first free block in queue test counter (if it exceed