MINOR: stream-int: implement a new stream_int_update() function

MINOR: stream-int: implement a new stream_int_update() function

This function is designed to be called from within the stream handler to
update the channels' expiration timers and the stream interface's flags
based on the channels' flags. It needs to be called only once after the
channels' flags have settled down, and before they are cleared, though it
doesn't harm to call it as often as desired (it just slightly hurts
performance). It must not be called from outside of the stream handler,
as what it does will be used to compute the stream task's expiration.

The code was taken directly from stream_int_update_applet() and
stream_int_update_conn() which had exactly the same one except for
applet-specific or connection-specific status update.