srv_shutdown(): Do not call log_free_check(), because it will now
be repeatedly called by ibuf_merge_all(). Do not call
srv_sync_log_buffer_in_background(), because we do not actually care
about durability during shutdown. Log writes will already be triggered
by buf_flush_page_cleaner() for writing back modified pages, possibly by
log_free_check().

logs_empty_and_mark_files_at_shutdown(): Clean up a condition.
This function is the caller of srv_shutdown(), and it will ensure that
the log and the buffer pool will be in clean state before shutdown.