Get all non-expired revealed namespaces.

def namedb_get_all_revealed_namespace_ids( self, current_block ):
    """
    Get all non-expired revealed namespaces.
    """
    
    query = "SELECT namespace_id FROM namespaces WHERE op = ? AND reveal_block < ?;"
    args = (NAMESPACE_REVEAL, current_block + NAMESPACE_REVEAL_EXPIRE )

    namespace_rows = namedb_query_execute( cur, query, args )
    ret = []
    for namespace_row in namespace_rows:
        ret.append( namespace_row['namespace_id'] )

    return ret