InnoDB read-ahead may cause page writes

The following stack trace could cause reduced performance in cases when the speculative read-ahead is futile, and some useful pages are evicted as a result:

buf_LRU_get_free_block

buf_page_init_for_read

buf_read_page_low

buf_read_page_background

btr_cur_prefetch_siblings

It could be more useful to only attempt read-ahead if a free block is available in the buffer pool without evicting any page that:

1. is not marked as freed, or
2. is not marked as dirty

Furthermore, we should probably avoid read-ahead altogether for temporary tables.