MINOR: stconn: flag the stream endpoint descriptor when the app has started

MINOR: stconn: flag the stream endpoint descriptor when the app has started

In order to improve our ability to distinguish operations that had
already started from others under high loads, it would be nice to know
if an application layer (stream) has started to work with an endpoint
or not. The use case typically is a frontend mux instantiating a stream
to instantly cancel it. Currently this info will take some time to be
detected and processed if the applcation's task takes time to wake up.
By flagging the sedesc with SE_FL_APP_STARTED the first time a the app
layer starts, the lower layers can know whether they're cancelling a
stream that has started to work or not, and act accordingly. For now
this is done unconditionally on the backend, and performed early in the
only two app layers that can be reached by a frontend: process_stream()
and process_hstream() (for haterm).