Add attribute uniqueness plugin implementation that provides single-server
attribute uniqueness. The plugin has the following features:

- provides ability to specify a group of attribute types that must have
  unique values; if no attribute types are specified then the plugin allows
  the operations to proceed with no checking

- provides ability to specify a set of base DNs that limit the scope of the
  uniqueness checking; if no base DNs are specified the server's public
  naming contexts are used

- allow changing of these configuration options without server restart

- allows the uniqueness checking to span multiple base DNs; if the server's
  public naming contexts are used, then the specified attribute type values must
  be globally unique within the server


Two configuration attributes have been added:

1. ds-cfg-unique-attribute-type used to specify the unique attribute type(s)
2. ds-cfg-unique-attribute-base-dn used specify the base DN(s) to limit the search scope

A disabled plugin configuration has been added to the config.ldif file for the uid attribute:

dn: cn=UID Unique Attribute ,cn=Plugins,cn=config
objectClass: top
objectClass: ds-cfg-plugin
objectClass: ds-cfg-unique-attribute-plugin
cn: UID Unique Attribute
ds-cfg-plugin-class: org.opends.server.plugins.UniqueAttributePlugin
ds-cfg-plugin-enabled: false
ds-cfg-plugin-type: preOperationAdd
ds-cfg-plugin-type: preOperationModify
ds-cfg-plugin-type: preOperationModifyDN
ds-cfg-unique-attribute-type: uid

Issue 258.