DFS: Fixed wrong scope in stream file listener

Request listeners are all initialized at the same time, and the siteaccess listener is one of them.
This means that we can't rely on dynamic parameters, and must use the ConfigResolver directly
to get the io url_prefix.

Furthermore, the image legacy io service has to be made lazy, or else the prefixes it uses will be
reset before the ConfigResolver is configured.
Merge pull request #1041 from ezsystems/dfs_stream_listener_prefix_scope

DFS: Fixed wrong scope in stream file listener