hash index at shutdown

btr_search_disable(): Just drop the entire adaptive hash index,
without dropping every record separately.

buf_pool_clear_hash_index(): Renamed and simplified from
buf_pool_drop_hash_index(). Set block->index = NULL for every block in
the buffer pool. Do not release the btr_search_latch. The caller will
have to adjust other data structures.

Remove block->is_hashed. It is redundant, should be always equal to
block->index != NULL.

Remove btr_search_fully_disabled, btr_search_enabled_mutex, and
SYNC_SEARCH_SYS_CONF. We drop the AHI in one pass, without releasing
the btr_search_latch in between.

Replace void* with const rec_t* and add assertions on btr_search_latch
and btr_search_enabled to ha0ha.h, ha0ha.ic, ha0ha.c.

page_set_max_trx_id(): Ignore the adaptive hash index. I forgot to
push this in rb:750.

btr0sea.c: Always after acquiring btr_search_latch, check for
block->index==NULL or !btr_search_enabled. We can now set
block->index=NULL while only holding btr_search_latch in exclusive
mode. Always acquire btr_search_latch before reading block->index,
except in shortcuts when testing for block->index == NULL.

ha_clear(), ha_search(): Unused function, remove.

buf_page_peek_if_search_hashed(): Remove. This function may avoid
latching a page at the cost of doing a duplicate buf_pool->page_hash
lookup.

rb:775 approved by Inaam Rana