Add tiny core class extension for hash subsets.
Allow webmachine to handle disconnected clients.

Note that this method is called after unsuccessful response was
performed as a result of client disconnection.
Fix core ext argument error.
Remove obsolete code.

This code served for stream-from-file-like-tail concept which is no
more.
Unsubscribe clients after we know they're disconnected.
Subscribe incoming client for event updates stream.

Yet another subscribe... I hope this is the last iteration.
Funny how finish_request hack enabled partial streaming.
Cleanup unused code.