TEIIDDES-305: Adds in statistics for performance optimisation

* To optimise the query, the distinct value and null value counts are
  required. These properties can be set using the searches already
  conducted when displaying the child attributes in the column page tree

* Adds fields to LdapAttributeNode for the null value count, distinct value
  count and maximum length of the attribute's values.
Merge pull request #243 from phantomjinx/ldap-wizard

TEIIDDES-305: Adds in statistics for performance optimisation