hash index, because it might occupy the chunk we would like to free.

TODO: In btr_search_check_free_space_in_heap(), release the block if
btr_search_latch is not immediately available.

buf_pool_shrink(): Split from buf_pool_resize().

btr_search_disabled: New variable, similar to srv_use_adaptive_hash_indexes
that was removed earlier.

btr_search_disable(): New function: disable and purge the adaptive hash index.

btr_search_enable(): New function: enable the adaptive hash index.

ha_clear(): New function: Empty a hash table and free the memory heaps.