row_drop_tables_for_mysql_in_background(): Copy the table name
before closing the table handle, to avoid heap-use-after-free if
another thread succeeds in dropping the table before
row_drop_table_for_mysql_in_background() completes the table name lookup.

dict_mem_create_temporary_tablename(): With innodb_safe_truncate=ON
(the default), generate a simple, unique, collision-free table name
using only the id, no pseudorandom component. This is safe, because
on startup, we will drop any #sql tables that might exist in InnoDB.
This is a backport from 10.3. It should have been backported already
as part of backporting MDEV-14717,MDEV-14585 which were prerequisites
for the MDEV-13564 backup-friendly TRUNCATE TABLE.
This seems to reduce the chance of table creation failures in
ha_innobase::truncate().

ha_innobase::truncate(): Do not invoke close(), but instead
mimic it, so that we can restore to the original table handle
in case opening the truncated copy of the table failed.