Add variable to define where to store temporary files created while collecting engine-independent statistics

When engine-independent table statistcis is estimated by ANALYZE like this:

ANALYZE TABLE t FOR COLUMNS (c1, c2, ...) INDEXES(i1,i2, ...);

huge temporary files may be created in tmpdir that is used for other temporary objects as well:

lsof -p 25338 | grep "/mysql_tmp"

...

mysqld 25338 mysql 742u REG 253,7 8819310592 51 /mysql_tmp/MYUxFeMC (deleted)

InnoDB already provide a variable to store its temporary files separately (innodb_tmpdir), but surely it is ignored when engine-independent statistcis is estimated.

I think it makes sense to add new variable, like stats_tmpdir, that let to define separate place for temporary files created during ANALYZE.