Per-thread memory usage statistics
Base code and idea from comes from
http://mysql.taobao.org/index.php/Patch_source_code#per-thread_memory_usage_statistics.

User visiable changes:

• information_schema.processlist has a new columns: MEMORY_USAGE
• SHOW STATUS has a new variable: Memory_used.

The idea is that we mark all memory that are thread specific with MY_THREAD_SPECIFIC.
Memory counting is done per thread in the my_malloc_size_cb_func callback function from my_malloc().

The following changes in interfaces was needed to get this to work:

• init_alloc_root() amd init_sql_alloc() has extra option so that one can mark memory with MY_THREAD_SPECIFIC
• One now have to use alloc_root_set_min_malloc() to set min memory to be allocated by alloc_root()
• my_init_dynamic_array() has extra option so that one can mark memory with MY_THREAD_SPECIFIC
• my_net_init() has extra option so that one can mark memory with MY_THREAD_SPECIFIC
• Added flag for hash_init() so that one can mark hash table to be thread specific.
• Added flags to init_tree() so that one can mark tree to be thread specific.
• Added flag to Warning_info::Warning_info() if the structure should be fully initialized.
• String elements can now be marked as thread specific.

Other things:

• Internal HEAP tables are now marking it's memory as MY_THREAD_SPECIFIC.