api: a stream listens for its token's end from before it is answered

Review 4, F9: a stream answered to a token subscribed to the document's changes only
when its body was first polled, and looked at the token only after a change came: one
revoking it that came between the request's check and the subscription (a node
applying what it was behind on) went unseen until the next change of protocol data, or
the token's expiry.

Now `streams_end` subscribes before the answer is made, and the token is looked at once
as the stream begins (`refused_now`, the document and this node's ended sessions), then
with every change of protocol data as before. A request that isn't answered with a
stream drops its subscription with the answer.

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