heap/hp_create.c:
  limit HEAP table size with max_heap_table_size
heap/hp_write.c:
  limit HEAP table size with max_heap_table_size
include/heap.h:
  limit HEAP table size with max_heap_table_size
sql/ha_heap.cc:
  limit HEAP table size with max_heap_table_size
  better estimation for mem_per_row