MINOR: stream-int: implement the stream_int_notify() function

MINOR: stream-int: implement the stream_int_notify() function

stream_int_notify() was taken from the common part between si_conn_wake_cb()
and si_applet_done(). It is designed to report activity to a stream from
outside its handler. It'll generally be used by lower layers to report I/O
completion but may also be used by remote streams if the buffer processing
is shared.