Use NodeSet or set instead of keys list in Task.iter_*()

In a process to simplify (and improve the performance of) the library API, we are going to change the ''keys'' list to a NodeSet whenever possible (will failback to standard ''set'' otherwise) for the following [wiki:Task] methods:

{{{iter_buffers()}}}
{{{iter_errors()}}}
{{{iter_retcodes()}}}
