(ITS#8239) back-meta crashes after deleting a rewrite rule

[ryan@openldap.org]

Full_Name: Ryan Tandy
Version: master
OS: Debian
URL: 
Submission from: (NULL) (24.68.37.4)
Submitted by: ryan


Deleting rewrite rules from a back-meta target is not implemented correctly.

Deleting a single value (vs the entire attribute) isn't checked for, the entire
rewrite_info and list of saved rules is always freed.

Then meta_back_conn_destroy doesn't notice the rewrite_session has already been
cleaned up, and tries to free it again.