adaptive hash index for a B-tree page.  (Bug #16582)

btr_search_drop_page_hash_index(): Retry the operation if a hash index
with different parameters was built meanwhile.  Add diagnostics for the
case that hash node pointers to the page remain.  This fix is from Heikki.

btr_search_info_update_hash(), btr_search_info_update_slow():
Document the parameter "info" as in/out.