This is part of Lightstreamer communication protocol and I confirm that it is something that is not documented, as not part of the public interface with the application.
In particular, STREAMING_IN_PROGRESS is related with HTTP streaming, but also with HTTP long polling, and continuous requests are consistent with a polling scenario; if realtime data is frequent, long polling may become very short.
Note that this kind of requests is only used by old versions of the client libraries, or when old versions of browsers are involved. Can you confirm this?