LDAP authenticator should rebind to proxy user after validating user credentials

In LDAP authenticator a admin user can be used to connect to LDAP server and validate user credential etc. That is used when users don't have right to list groups and things like that to be able to map LDAP and XWiki membership.

By default the way to validate user credential is by using LDAP bind, so everything done after that is done using user rights instead of admin user rights. The authenticator should rebind to admin user after user credential validation.