Commit plugin for maintaining referential integrity. Issue 257.

New configuration  attributes:

- ds-cfg-referential-integrity-attribute-type Specify attribute types that referential integrity will be checked on; this is a mandatory attribute

-ds-cfg-referential-integrity-base-dn Specify base DN that will limit scope of reference check; if not specified the server's public naming contexts are used

- ds-cfg-referential-integrity-update-interval Specify update interval for background referential integrity processing; if update interval > 0 plugin performs background processing; default is 0

-ds-cfg-referential-integrity-log-file Specify log file location for update records when background processing is enabled; default is <instance>/logs/referint

The plugin is disabled by default:

dn: cn=Referential Integrity,cn=Plugins,cn=config
objectClass: top
objectClass: ds-cfg-plugin
objectClass: ds-cfg-referential-integrity-plugin
cn: Referential Integrity
ds-cfg-plugin-class: org.opends.server.plugins.ReferentialIntegrityPlugin
ds-cfg-plugin-enabled: false
ds-cfg-plugin-type: postOperationDelete
ds-cfg-plugin-type: postOperationModifyDN
ds-cfg-plugin-type: subordinateModifyDN
ds-cfg-referential-integrity-attribute-type: member
ds-cfg-referential-integrity-attribute-type: uniqueMember