buf_LRU_make_block_young(): Merge with buf_page_make_young().

buf_pool_check_no_pending_io(): Remove. Replaced with
buf_pool.any_io_pending() and buf_pool.io_pending(),
which do not unnecessarily acquire buf_pool.mutex.

buf_pool_t::init_flush[]: Use atomic access, so that
buf_flush_wait_LRU_batch_end() can avoid acquiring buf_pool.mutex.

buf_pool_t::try_LRU_scan: Declare as bool.