Implement a new kind of directory for LDAP connections

The current LDAP Directory implementation may suffer from LDAP or network problems/interruptions.

One solution would be to implement a new directory that stores in a local SQL cache the values already fetched from the LDAP.
Its mechanism would be to first query the LDAP, and if an error (Socket Exception for instance) occurs, then we try to read the value in a local cache.

We need to determine how the values will expire.