Use RBTDB instead of internal LDAP cache.
    
    Internal RBTDB is created for each zone and most calls from
    internal database API are relayed to the internal RBTDB.
    
    Each RBTDB initially contains fake SOA and NS records and
    the content of the internal RBTDB is synchronized with LDAP
    during start-up.
    Responses retured to clients during initial synchronization
    can be incomplete and can contain illegal SOA and NS records.
    
    DNS updates are not propagated to LDAP and SOA serial number is not
    maintained at all.
    
    LDAP MODRDN operation is not supported because current code lacks
    LDAP entryUUID->DNS name mapping database.
    
    Side-effect of transition to RBTDB is that DNS wildcards are supported
    from now. Another side-effect is that non-existing records
    are virtually 'cached' because RBTDB is authoritative source
    of information. As a result, read-performance is nearly same as
    with plain BIND 9.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/95
    https://fedorahosted.org/bind-dyndb-ldap/ticket/6
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>