Fixed to build with the recent Native Framework.
exec replies fixed

Reply is the same as any other elliptics command, it should be
performed asynchronously, and cocaine response object should be held
alive until reply will actually complete
queue: session holding on node changed to weak_ptr, must conform to that
queue: updated python scripts to work with newest python binding

plus nodes.py:
- switched to use stock elliptics.create_node
- added wrapper to tie up lifetime of session and underlying node
queue_stats.py: updated to python api changes

- new and old exec_ instead of exec_event
- using None to send event to all nodes in the group
queue_stats.py: revert using None to fanout exec to all nodes

exec_context of the individual result does not carry information
about address and src_id of the node which produces that result, so its
impossible to differentiate one node from another.