Add rebuild task to memberof plugin
    
    This task allows us to rebuild memberuid and memberof attributes throughout the
    database. This way we can upgrade from version 0.4 databases that didn't
    generate and store memberuid.
    The task can be invoked by adding a speaicl named entry to the ldb file.
    The entry dn to use is: @MEMBEROF-REBUILD, the entry has no attributes and any
    attribute is ignored at present.
    The entry will not be stored in the database but will just trigger the task to
    execute a rebuild of the memberof and memberuid attributes