Now that InnoDB startup drops tables whose name starts with #sql,
the table ID alone should be enough to create a unique table name.

dict_temp_file_num, dict_mem_init(): Remove.

dict_mem_create_temporary_tablename(): Use a combination of
"#sql-ib" and the ID of the table that is being rebuilt
in TRUNCATE TABLE, or discarded in the commit of table-rebuilding
ALTER TABLE...ALGORITHM=INPLACE, or deferred to the DROP queue.

Thanks to Sergey Vojtovich for pointing this out.