Caching user DNs  This property caches user DNs (distinguished
names) locally when derby.authentication.provider is set to LDAP
and derby.authentication.ldap.searchFilter is set to derby.user.
When you provide a user DN with this property, Derby is able to
avoid an LDAP search for that user's DN before authenticating.
For those users without DNs defined with this property, Derby
performs a search using the default value of
derby.authentication.ldap.searchFilter.