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.
This is dicey, because half-closed (local) vs. half-closed (remote) is a distinction of sender/recipient.  The fuzziness existed before, but it's now highlighted because of the stream states being explicitly named.  (BTW, does that mean a client can legitimately send a PUSH_PROMISE on a pushed stream?  It is peer-initiated, after all...!)