Identifying streams more precisely

Two issues were raised about the use of the words "existing" and "current" to refer to streams.  These were a little squishy, so I tightened the language a little.  The text no longer uses either word to refer to streams, instead using stream states to describe where the relevant text applies and favouring "identified" as a fallback.
Same as before, though it's less of an issue here since you're talking specifically about managing the flow control windows of streams on which you could still send.