add new ldap functions that deal directly with the ldap entry locally

[davidcoutadeur]

This issue is linked with ltb-project/service-desk#214

This is a proposition for adapting Ltb/Directory.php functions.

We could rewrite some functions for not calling ldap operations, but treating directly a previously returned entry.

For example, we could create a new isLocked function that receives the local entry to treat in argument, and modify the current isLocked function to call the newer.

Same for isAccountValid, isAccountEnabled, isPasswordExpired