mem_heap_get_size() scans all allocated blocks to calculate the total
size of the heap. This patch introduces a new, total_size, field in
mem_block_info_struct. This field is valid only for base block 
(i.e.: the first block allocated for the heap) and is set to
ULINT_UNDEFINED in other blocks.
This considerably improves the performance of redo scan during recovery.

rb://108 issue#216

Approved by: Heikki