Not all LDAP servers provide LastModification Dates

[alexeiser]

The Google Secure LDAP feature does not provide a last modification date attribute to records, which is required by nsscache to use an ldap source.

While the change to make LastModification option is straightforward it implies that a full sync is required every time. Before providing a diff that handles a missing LastModification date, I wanted to check if it was desired.

NOTE: I have worked around it locally by creating a python LDAP client that generates the S3 and HTTP outputs that can then be used with the existing sources.