buf_page_t: Note that the LRU fields are protected by buf_pool_mutex
only, not block->mutex or buf_pool_zip_mutex.

buf_page_get_freed_page_clock(): Note that this is sometimes invoked
without mutex protection.

buf_pool_get_oldest_modification(): Note that the result may be out of
date.

buf_page_get_LRU_position(), buf_page_is_old(): Assert that the buffer
pool mutex is being held.

buf_page_release(): Assert that dirty blocks are in the flush list.