btr_search_disabled: Rename to btr_search_enabled and change the type
to char, so that it can be directly linked to the MySQL parameters.
Note that the variable is protected by btr_search_latch and
btr_search_enabled_mutex, a new mutex introduced in this patch.

btr_search_enabled_mutex: A new mutex, to protect btr_search_enabled
together with btr_search_latch.

buf_pool_drop_hash_index(): New function, to be called from
btr_search_disable().

btr_search_disable(), btr_search_enable(): Fix bugs.  These functions
were previously unused.

btr_search_guess_on_hash(), btr_search_build_page_hash_index():
Check btr_search_enabled once more, while holding btr_search_latch.

btr_cur_search_to_nth_level(): Note that the reads of btr_search_enabled
may be dirty and explain why it should not be a problem.

innobase_adaptive_hash_index: Remove. The variable btr_search_enabled will be used directly instead.

innodb_adaptive_hash_index_update(): New function, an update callback for
innodb_adaptive_hash_index.  This will call either btr_search_disable()
or btr_search_enable() when the value is assigned.  The functions will
be called even if the value does not appear to be changed, e.g., when
setting from TRUE to TRUE or FALSE to FALSE.

rb://85 approved by Heikki Tuuri.  This addresses Issue #163.