MEDIUM: stream-int: use the same stream notification function for applets and conns

MEDIUM: stream-int: use the same stream notification function for applets and conns

The code to report completion after a connection update or an applet update
was almost the same since applets stole it from the connection. But the
differences made them hard to maintain and prevented the creation of new
functions doing only one part of the work.

This patch replaces the common code from the si_conn_wake_cb() and
si_applet_wake_cb() with a single call to stream_int_notify() which only
notifies the stream (si+channels+task) from the outside.

No functional change was made beyond this.