Issue 45 - RFE - Allow to rebuild the compat tree
    
    Description:
    	For different reasons, the compat tree maps might get corrupted and some entries would be missing.
    	This RFE provides a way to rebuild online the whole compat tree on demand via a DS task.
    
    Fix:
    	It registers tasks callbacks under cn=chema compatibility
    	refresh task,cn=tasks,cn=config. The task meta data contains
            the filter (SCH_CONTAINER_CONFIGURATION_FILTER) and the
            plugin_state (not retrievable via pblock).
            The rebuild task (backend_shr_refresh_thread) is shared
            as it can be used by NIS or Schema Compat.
            The rebuild task disable access to the maps, free and
    	reinit them. Then rebuild them using the already implemented
            backend_shr_data_initialize_thread.
    
    Review: Alexander Bokovoy (thanks !)