The flushing of the InnoDB temporary tablespace is unnecessarily
tied to the write-ahead redo logging and redo log checkpoints,
which must be tied to the page writes of persistent tablespaces.

Let us simply omit any pages of temporary tables from buf_pool.flush_list.
In this way, log checkpoints will never incur any 'collateral damage' of
writing out unmodified changes for temporary tables.

After this change, pages of the temporary tablespace can only be written
out by buf_flush_lists(n_pages,0) as part of LRU eviction. Hopefully,
most of the time, that code will never be executed, and instead, the
temporary pages will be evicted by buf_release_freed_page() without
ever being written back to the temporary tablespace file.

This should improve the efficiency of the checkpoint flushing and
the buf_flush_page_cleaner thread.

Reviewed by: Vladislav Vaintroub