DB's rootDN independently of the configured back-config rootDN.

Fix syncprov_findcsn to set unchecked limit. (ITS#3939)

streamline back-ldap/slapo-chain configuration; add the possibility to cache non-configured, anonymous referral URIs

fix dangling resources issue in slapd-ldap; completely rework slapo-chain to fix the resource leak/concurrency issue; add support for multiple well-known URIs to set credentials for, and deal with unknown URIs anonymously; similar reworking and cleanup for slapd-meta