Protocols.LDAP: Lock on AD-server (DNS round robin) until failover needed to avoid flip-flop.
Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=7406
Reported by @grubba
[roxen.com #23861], [roxen.com #20751]:
If AD import is configured to an URL that resolves to several ip numbers the user import may fail.
This is due to the incremental update relying on uSNChanged, which is unique for each of the LDAP servers in the cluster.
The Protocols.LDAP module should attempt to connect to the same server in the cluster every time and only switch servers when the old one fails.